MSYS: use cp -f for QMAKE_INSTALL_*

... in mingw-using specs because msysgit
doesn't provide install.exe and Windows
doesn't care about Unix permissions anyway.

Task-number: QTBUG-31147

Change-Id: Ic8032ca1a970ef41381852b6c5c372b805a124f1
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>