Normalize execution guards on OStatus php files; mostly helps cut down on annoying 'class not found' errors when something spiders the dirs. :P

plugins/OStatus/classes/Ostatus_source.php

@@ -17,6 +17,10 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+if (!defined('STATUSNET')) {
+    exit(1);
+}
+
 /**
  * @package OStatusPlugin
  * @maintainer Brion Vibber <brion@status.net>