FFmpeg: Re-enable shared libraries

Removing shared libraries from the FFmpeg build has broken a couple of formulae
that depend on it. This is because the shared libraries provided linker
information pertaining to the FFmpeg dependencies.

Re-enabling shared libraries fixes build issues FFmpeg-PHP (issue Homebrew/homebrew#6832) and
GRASS GIS.

Static libraries are still built.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Formula/ffmpeg.rb

@@ -19,6 +19,7 @@ class Ffmpeg < Formula
 
   def install
     args = ["--prefix=#{prefix}",
+            "--enable-shared",
             "--enable-gpl",
             "--enable-version3",
             "--enable-nonfree",