KBuildSycoca: always process application desktop files.

If nothing else had changed, and we had no application desktop files
in the existing ksycoca, the vfolder code was skipped and ksycoca wasn't
updated. Granted, this can only happen on a virgin environment like
CI where the xdg data dirs have no apps installed.

This should fix "'fakeApplicationService' returned FALSE" in
KMimeAssociationsTest.