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 TLS/SSL support and attempting to play a stream over HTTPS would result in generic ffmpeg 'Could not open file' and 'Error creating demuxer' error messages.

Here's a couple of reference tickets:
andybotting/xbmc-addon-plus7#8
andybotting/xbmc-addon-plus7#9

This commit fixes that build order, and XBMC now builds its ffmpeg with gnutls as an external library, and my custom addon that streams HLS over HTTPS now works. 

I've verified this in a x86_64 Generic build, and I'm now in the process of building a RPi version.