v4l2object: fix extra-controls leak

Gst struct v4l2object->extra_controls is created if user sets appropriate
option but it is not freed on destruction of v4l2object.

https://bugzilla.gnome.org/show_bug.cgi?id=773580

sys/v4l2/gstv4l2object.c

@@ -519,6 +519,10 @@ gst_v4l2_object_destroy (GstV4l2Object * v4l2object)
     gst_caps_unref (v4l2object->probed_caps);
   }
 
+  if (v4l2object->extra_controls) {
+    gst_structure_free (v4l2object->extra_controls);
+  }
+
   g_free (v4l2object);
 }