qt_add_protobuf() broken in 6.6.0-beta3 for VS2019 build system

The attached tarball build succeeds when using Ninja in windows, but fails with the following errors when using the VS2019-project-based build system.

Seems like there's something amiss in the protobuf-library cmake build generation stuff.

```
17:13:31: Running steps for project test_timestamp_proto...
17:13:31: Starting: "C:\Qt\Tools\CMake_64\bin\cmake.exe" --build C:/Users/GlenM/src/build-test_timestamp_proto-Desktop_Qt_6_6_0_MSVC2019_64bit-Debug --target ALL_BUILD --config Debug
Microsoft (R) Build Engine version 16.11.2+f32259642 for .NET Framework
Copyright (C) Microsoft Corporation. All rights reserved.

Generating QtProtobuf timestamplib sources for qtprotobufgen...
CUSTOMBUILD : CMake warning : [C:\Users\GlenM\src\build-test_timestamp_proto-Desktop_Qt_6_6_0_MSVC2019_64bit-Debug\timestamplib_protobuf_registration_autogen.vcxproj]
No source or binary directory provided. Both will be assumed to be the
same as the current working directory, but note that this warning will
become a fatal error in future CMake releases.

CUSTOMBUILD : CMake error : The source directory "C:/Users/GlenM/src/build-test_timestamp_proto-Desktop_Qt_6_6_0_MSVC2019_64bit-Debug" does not appear to contain CMakeLists.txt. [C:\Users\GlenM\src\build-test_timestamp_proto-Desktop_Qt_6_6_0_MSVC2019_64bit-Debug\timestamplib_protobuf_registration_autogen.vcxproj]
Specify --help for usage, or press the help button on the CMake GUI.
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Microsoft\VC\v160\Microsoft.CppCommon.targets(241,5): error MSB8066: Custom build for 'C:\Users\GlenM\src\build-test_timestamp_proto-Desktop_Qt_6_6_0_MSVC2019_64bit-Debug\CMakeFiles\f4538148952143334a7581c1d863e7dd\timestamp.qpb.h.rule;C:\Users\GlenM\src\build-test_timestamp_proto-Desktop_Qt_6_6_0_MSVC2019_64bit-Debug\CMakeFiles\8c04ee2006941954f9249e07587c6666\timestamplib_protobuf_registration_autogen.rule' exited with code 1. [C:\Users\GlenM\src\build-test_timestamp_proto-Desktop_Qt_6_6_0_MSVC2019_64bit-Debug\timestamplib_protobuf_registration_autogen.vcxproj]
Generating QtProtobuf timestamplib sources for qtprotobufgen...
CUSTOMBUILD : CMake warning : [C:\Users\GlenM\src\build-test_timestamp_proto-Desktop_Qt_6_6_0_MSVC2019_64bit-Debug\timestamplib_qtprotobufgen_deps_0.vcxproj]
No source or binary directory provided. Both will be assumed to be the
same as the current working directory, but note that this warning will
become a fatal error in future CMake releases.

CUSTOMBUILD : CMake error : The source directory "C:/Users/GlenM/src/build-test_timestamp_proto-Desktop_Qt_6_6_0_MSVC2019_64bit-Debug" does not appear to contain CMakeLists.txt. [C:\Users\GlenM\src\build-test_timestamp_proto-Desktop_Qt_6_6_0_MSVC2019_64bit-Debug\timestamplib_qtprotobufgen_deps_0.vcxproj]
Specify --help for usage, or press the help button on the CMake GUI.
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Microsoft\VC\v160\Microsoft.CppCommon.targets(241,5): error MSB8066: Custom build for 'C:\Users\GlenM\src\build-test_timestamp_proto-Desktop_Qt_6_6_0_MSVC2019_64bit-Debug\CMakeFiles\f4538148952143334a7581c1d863e7dd\timestamp.qpb.h.rule;C:\Users\GlenM\src\build-test_timestamp_proto-Desktop_Qt_6_6_0_MSVC2019_64bit-Debug\CMakeFiles\8c04ee2006941954f9249e07587c6666\timestamplib_qtprotobufgen_deps_0.rule' exited with code 1. [C:\Users\GlenM\src\build-test_timestamp_proto-Desktop_Qt_6_6_0_MSVC2019_64bit-Debug\timestamplib_qtprotobufgen_deps_0.vcxproj]
17:13:31: The process "C:\Qt\Tools\CMake_64\bin\cmake.exe" exited with code 1.
Error while building/deploying project test_timestamp_proto (kit: Desktop Qt 6.6.0 MSVC2019 64bit)
When executing step "Build"
17:13:31: Elapsed time: 00:01.
```
reported_by_support_standard