package/ffmpeg: Add support for GnuTLS

Besides OpenSSL ffmpeg also supports GnuTLS.

If both GnuTLS and OpenSSL are enabled, the former takes precedence over
the latter:
http://git.videolan.org/?p=ffmpeg.git;a=blob;f=configure;h=7f9fed31a491ed0991aee2d354dba82e54e3291e;hb=refs/heads/release/2.8#l2719



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>