Unified multiple definitions of container_of() macro.

Removed duplicate definitions of the container_of() macro and
refactored sources to use the single implementation.

Signed-off-by: Jon A. Cruz <jonc@osg.samsung.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>

xwayland/launcher.c

@@ -36,6 +36,7 @@
 #include <signal.h>
 
 #include "xwayland.h"
+#include "shared/helpers.h"
 
 
 static int