mumble.pro: fix g15 logic for detecting Windows and OS X.
mumble.pro: fix g15 logic for detecting Windows and OS X.

The new G15 qmake logic accidently used a logical AND
instead of a logical OR.

The result was that neither OS X nor Windows would have any
G15 engines enabled. Oops.

Thanks to dekeonus for reporting this bug.

Fixes mumble-voip/mumble#1927

unascribed/mumble,austinliou/mumble,SuperNascher/mumble,LuAPi/mumble,SuperNascher/mumble,SuperNascher/mumble,SuperNascher/mumble,austinliou/mumble,LuAPi/mumble,austinliou/mumble,LuAPi/mumble,unascribed/mumble,austinliou/mumble,unascribed/mumble,Lartza/mumble,SuperNascher/mumble,Lartza/mumble,LuAPi/mumble,SuperNascher/mumble,unascribed/mumble,austinliou/mumble,LuAPi/mumble,austinliou/mumble,unascribed/mumble,Lartza/mumble,LuAPi/mumble,LuAPi/mumble,austinliou/mumble,Lartza/mumble,unascribed/mumble,Lartza/mumble,SuperNascher/mumble,unascribed/mumble,unascribed/mumble,austinliou/mumble,SuperNascher/mumble,LuAPi/mumble,SuperNascher/mumble,LuAPi/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble