qt5_add_big_resources: second pass never reruns

The custom command for second pass is generated with empty dependency (tested with msvc and ninja generators).

Changing the depends in Qt5CoreMacros.cmake:337 from "rcc_object_${outfilename}" to "$<TARGET_OBJECTS:rcc_object_${outfilename}>" can fix that.

 According to CMake document of add_custom_command:

DEPENDS

Specify files on which the command depends. Each argument is converted to a dependency as follows:

1. If the argument is the name of a target (created by the add_custom_target(), add_executable(), or add_library() command) a target-level dependency is created to make sure the target is built before any target using this custom command. Additionally, if the target is an executable or library, a file-level dependency is created to cause the custom command to re-run whenever the target is recompiled.

a file-level dependency should exist. So perhaps this is a bug in CMake generator. But we can explicitly add $<TARGET_OBJECTS:rcc_object_${outfilename}> to ensure this file-level dependency is generated.