spa: ffmpeg: remove libavformat dependency

`av_register_all()` is the only thing used from libavformat,
but that doesn't seem to be needed for codec enumeration,
so remove it, and call `avcodec_register_all()` instead
when libavcodec < 58.10.100.

spa/plugins/ffmpeg/meson.build

@@ -5,6 +5,6 @@ ffmpeg_sources = ['ffmpeg.c',
 ffmpeglib = shared_library('spa-ffmpeg',
                           ffmpeg_sources,
                           include_directories : [spa_inc],
-                          dependencies : [ avcodec_dep, avformat_dep ],
+                          dependencies : [avcodec_dep],
                           install : true,
 		          install_dir : spa_plugindir / 'ffmpeg')