mumble.pro: use forward slash for GENQRC also on Windows
mumble.pro: use forward slash for GENQRC also on Windows

This fixes a problem with MinGW on Linux where the "win32" block is triggered and the two backslashes are used, resulting in a path without slashes.
Python on Windows handles the path correctly with both methods.

Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble