Ensure that all .obj wait for the PCH to finish

Otherwise, you get compilation errors like:
c:\Qt\qt5\qtbase\src\corelib\global\qlogging.cpp : fatal error C1083: Cannot open precompiled header file: 'configure_pch.pch': Permission denied

Change-Id: I56401ce7f6aff32e97617c3b4fd5fe06fbeebf58
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>