contrib: Fix libgnurx to use mingw's ar.

Thanks to KonaB1end for help connecting the dots!



git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6469 b64f7644-9d1e-0410-96f1-a4d463321fa5
Branch "libgnurx" committed:

https://trac.handbrake.fr/changeset/6469
HandBrake/HandBrake@f4f93bd