flatpak: Enable obs-libfdk

Freedesktop runtime provides it.

CI/flatpak/com.obsproject.Studio.json

@@ -505,7 +505,8 @@
         "-DENABLE_JACK=ON",
         "-DENABLE_RTMPS=ON",
         "-DENABLE_VLC=OFF",
-        "-DENABLE_AJA=ON"
+        "-DENABLE_AJA=ON",
+        "-DENABLE_LIBFDK=ON"
       ],
       "secret-opts": [
         "-DRESTREAM_CLIENTID=$RESTREAM_CLIENTID",