configure: select buffersink_filter when ffmpeg is enabled

buffersink_filter is a strong requirement for compiling ffmpeg.
Fixes ffmpeg compilation with --disable-everything.

configure

@@ -1515,7 +1515,7 @@ postproc_deps="gpl"
 
 # programs
 ffmpeg_deps="avcodec avformat swscale"
-ffmpeg_select="buffer_filter"
+ffmpeg_select="buffer_filter buffersink_filter"
 ffplay_deps="avcodec avformat swscale sdl"
 ffplay_select="rdft"
 ffprobe_deps="avcodec avformat"