Require --enable-nonfree flag for libvpx.

The license of libvpx is incompatible with the (L)GPL.  As long as this is
the case, the only way to use it is by marking the result as nonfree.

Originally committed as revision 23371 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -2341,6 +2341,7 @@ die_license_disabled gpl postproc
 die_license_disabled gpl x11grab
 
 die_license_disabled nonfree libfaac
+die_license_disabled nonfree libvpx
 
 die_license_disabled version3 libopencore_amrnb
 die_license_disabled version3 libopencore_amrwb