[REG 3.2.0.75 -> 3.2.0.93] Cannot build Qt Designer Custom Widget

1. Create a new Qt Designer Custom Widget using the addin's wizard.
   Make sure you use Qt/MSBuild. For everything else use defaults.
2. Press Ctrl+B to build the project.
   This will fail with hundreds of build errors.

This is a regression. Version 3.2.0.75 could still build the project without any problems. It also builds fine when using CMake.

Found and tested by Squish test tst_new_project_file_creation.