multimedia/ffmpeg{,26}: make GNUTLS and OPENSSL mutually exclusive

[xmj]

Summary

building ffmpeg in poudriere (with some stale options), I discovered that
ffmpeg (and, similarly, ffmpeg26) can't be built with having both OPENSSL and
GNUTLS on.

Hence, make both ports define an OPTIONS_SINGLE for this case.

Test Plan

http://xmj.me/freebsd/buildlogs/ffmpeg-2.3.6_2,1.log
http://xmj.me/freebsd/buildlogs/ffmpeg26-2.6.1_1.log

[riggs]

Go ahead, commit this.