W32: Include rc file by its absolute name

Otherwise you get something like this at compile time:
windres -i gedit/gedit.rc --input-format=rc -o gedit/gedit-res.o -O coff
i686-w64-mingw32-gcc: error: gedit/gedit.rc: No such file or directory
i686-w64-mingw32-gcc: warning: '-x c' after last input file has no effect
i686-w64-mingw32-gcc: fatal error: no input files

https://bugzilla.gnome.org/show_bug.cgi?id=727658