lrelease.pri: add Windows-specific code path, to fix nmake build.
lrelease.pri: add Windows-specific code path, to fix nmake build.

NMake doesn't like Unix-style paths, whereas jom doesn't care.
This lead to a situation where AppVeyor builds (that use jom)
succeeded, but Jenkins builds (that use nmake) failed.

This commit fixes that by adding a Windows-specific codepath to
lrelease.pri that finds a Windows-style path for lrelease.exe.

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