ffmpeg: use linked_keg when checking optional deps

ffmpeg when built as head checks if x264 is installed using
`Formula.factory.installed?` that won't find x264 stable as it looks for a HEAD
build.  Switch from `.installed?` to `linked_keg.exist?` for all dependencies
as this will find them whether they are installed as head or stable.

Fixes #11996.
Closes #12020.

Signed-off-by: Charlie Sharpsteen <source@sharpsteen.net>

Library/Formula/ffmpeg.rb

@@ -48,7 +48,7 @@ def install
             "--enable-libfreetype",
             "--cc=#{ENV.cc}"]
 
-    args << "--enable-libx264" if Formula.factory('x264').installed?
+    args << "--enable-libx264" if Formula.factory('x264').linked_keg.exist?
     args << "--enable-libfaac" if Formula.factory('faac').installed?
     args << "--enable-libmp3lame" if Formula.factory('lame').installed?
     args << "--enable-librtmp" if Formula.factory('rtmpdump').installed?