[gnome-sdk-images] Add GstLibav using the default system ffmpeg

[Michael Catanzaro <mcatanzaro src gnome org>]

commit 7f3fd8329a16d5d3f4471b6c56ba0dd864785d0d
Author: Thibault Saunier <tsaunier igalia com>
Date:   Sun Mar 18 15:36:43 2018 -0300

    Add GstLibav using the default system ffmpeg

 org.gnome.Sdk.json.in | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)
---
diff --git a/org.gnome.Sdk.json.in b/org.gnome.Sdk.json.in
index 0a82798..75b66fe 100644
--- a/org.gnome.Sdk.json.in
+++ b/org.gnome.Sdk.json.in
@@ -557,6 +557,30 @@
                 }
             ]
         },
+        {
+            "name": "gstreamer-libav",
+            "build-options" : {
+                "arch" : {
+                    "i386" : {
+                        "config-opts" : [
+                            "--build=i586-unknown-linux-gnu"
+                        ]
+                    },
+                    "arm" : {
+                        "config-opts" : [
+                            "--build=arm-unknown-linux-gnueabi"
+                        ]
+                    }
+                }
+            },
+            "config-opts": ["--with-system-libav" ],
+            "sources": [
+                {
+                    "type": "git",
+                    "url": "https://anongit.freedesktop.org/git/gstreamer/gst-libav.git";
+                }
+            ]
+        },
         {
             "name": "libcanberra",
             "sources": [