[third_party/ffmpeg] make ffmpeg a shared library MTWN-68

This CL makes ffmpeg into a shared library.

Change-Id: I4f268eaba8a29773a3036316882a9dd4013d3d89
diff --git a/BUILD.gn b/BUILD.gn
index 37fcd5a..f66a3a3 100755
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -207,7 +207,7 @@
   }
 }
 
-source_set("ffmpeg") {
+shared_library("ffmpeg") {
   public_configs = [ ":ffmpeg_dependent_config" ]
   deps = [
     ":ffmpeg_internal",