fix warning when generating VS 2013 project

fix warning when generating VS 2013 project

/FS forces the compiler to synchronize pdb file writes.
This option is not needed when building with Visual Studio itself.
Still, qmake needs to know it when parsing the compiler flags.

Task-number: QTBUG-36535
Change-Id: Id5b68c4028844e0b95904e08b5121310a4ff13d6
Reviewed-by: Andy Shaw <andy.shaw@digia.com>
(cherry picked from commit qtbase/9b59e51c5064c67c423389c2c884d009903910c7)
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>