LinGui: fix linking with mingw

Fedora's mingw pthreads no longer has pthread_win32_process_attach_np,
so force linking to the pthreadw32 contrib package.


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6173 b64f7644-9d1e-0410-96f1-a4d463321fa5