Make .la file generation reproducible in qmake

qmake seems to be adding current date/time to the .la files for no
reason, so let's stop do that.

This way, two invocations of qmake actually gives bit for bit similar
output of .la files.

Change-Id: I93c7c4075cc1e05214849eec8629f41ce01e5914
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>