Support precompiled header files without file extension in VS projects

Having a precompiled header file without file extension (or without
common file extension) led to an incorrectly generated Visual
Studio project file. The custom build step for automatically
generating the corresponding source file was missing.
  Remove the file extension check that apparently was yet another
feeble attempt of runtime optimization.

Task-number: QTBUG-50442
Change-Id: I0552f94be12cbb70e2f32c242c7364699979bd81
Reviewed-by: Karsten Heimrich <karsten.heimrich@theqtcompany.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>