ffmpeg: add new branch 2.6.1 (no switching yet)

/cc maintainers @codyopel, @fuuzetsu.

Release notes don't indicate any new options or dependencies.
http://git.videolan.org/?p=ffmpeg.git;a=blob;f=RELEASE_NOTES;hb=release/2.6
APIchanges looks like we should be able to replace 2.5 by 2.6
without anything breaking (belongs to staging and needs some testing).

pkgs/development/libraries/ffmpeg/generic.nix

@@ -383,7 +383,7 @@ stdenv.mkDerivation rec {
     (mkFlag (libdc1394 != null && libraw1394 != null) "0.5" "libdc1394")
     (mkFlag (libiconv != null) "1.2" "iconv")
     #(mkFlag (libiec61883 != null && libavc1394 != null && libraw1394 != null) "1.0" "libiec61883")
-    (mkFlag (libmfz != null) "2.6" "libmfx")
+    #(mkFlag (libmfx != null) "2.6" "libmfx")
     (mkFlag (libmodplug != null) "0.9" "libmodplug")
     #(mkFlag (libnut != null) "0.5" "libnut")
     (mkFlag (libopus != null) "1.0" "libopus")