ffmpeg: fix build with gpl suboption and openssl

Openssl's license isn't compatible with GPL, so ffmpeg's configure script
only allows this combination if nonfree is enabled.

Fixes http://autobuild.buildroot.net/results/ed6/ed6129d584229ebe4a0b291bc6a1dd0be23cc92a/



Signed-off-by: Peter Korsgaard <peter@korsgaard.com>