Fix qmake's pro file cache to interact correctly with write_file

When writing a file with write_file() we have to inform the pro file parser
cache to discard the file if it's existant in the cache, to ensure that
calling include() after write_file() always works.

Change-Id: I7d09269a57de55ca30b0e11dd40770de9f919f64
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
(cherry picked from qtbase/f45d6414ad6ef1d75f049a61411cd954bcae29c3)
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>