externals: Add submodule ffmpeg

.gitmodules

@@ -37,3 +37,6 @@
 [submodule "opus"]
 	path = externals/opus/opus
 	url = https://github.com/xiph/opus.git
+[submodule "externals/ffmpeg"]
+	path = externals/ffmpeg
+	url = https://git.ffmpeg.org/ffmpeg.git

externals/ffmpeg

@@ -0,0 +1 @@
+Subproject commit 81c462ad95f143837a6a21126117515577f0977b