Fix ffmpeg build to include SSL support

The XBMC package didn't depend on gnutls, therefore it would be build
before gnutls. Because of this, XBMC's ffmpeg would be built without
and TLS/SSL support and break streaming from some secure streams.

In my case, it broke HLS streaming via a HTTPS url.
Merge pull request #3012 from andybotting/fix-ffmpeg-ssl

Fix ffmpeg build to include SSL support