Define STRIP for FFmpeg.

This will prevent the (ignored) build error "/bin/sh: 1: -wN: not found".

If the strip utility doesn't support the options -wN, there still will be
an (ignored) "strip: unrecognized option: -wN" error we currently can't do
anything about.


git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@36482 b3059339-0415-0410-9bf9-f77b7e298cf2