multimedia/ffmpeg: fix build with SDL option enabled

Since this is a patch that has already applied to the ffmpeg upstream,
we should either apply this patch or upgrade to an fixed upstream
version, see:

https://git.ffmpeg.org/gitweb/ffmpeg.git/commitdiff/e5163b1d3438

This issue has existed for some time and despite the port being
upgraded in between, therefore I'll commit the patch under the
"just fix it" exception granted by portmgr, it can be removed by
the maintainers when ffmpeg is upgraded to a fixed version.

PR: 266299
Reported by: m.ne@gmx.net (Martin Neubauer)
Approved by: portmgr (implicit)