Build and install libcanberra

gbrainy uses this to play sounds. Without it you see the following on
stderr when gbrainy tries to play a sound:

    Unix.PlaySound. Error System.DllNotFoundException: libcanberra-gtk3.so.0 assembly:<unknown assembly> type:<unknown type> member:(null)
      at (wrapper managed-to-native) gbrainy.Core.Platform.Unix.ca_gtk_play_for_widget(intptr,uint,string,string,string,string,intptr)
      at gbrainy.Core.Platform.Unix.PlaySound (System.IntPtr widget, System.String filename) [0x00000] in <c1cbe592d22c4597a90c664ab477780f>:0

org.gnome.gbrainy.json

@@ -39,6 +39,7 @@
             ]
         },
         "shared-modules/intltool/intltool-0.51.json",
+        "shared-modules/libcanberra/libcanberra.json",
         {
             "name": "mono",
             "buildsystem": "simple",