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>

tests/ivi_layout-test-plugin.c

@@ -35,6 +35,7 @@
 #include "weston-test-server-protocol.h"
 #include "ivi-test.h"
 #include "ivi-shell/ivi-layout-export.h"
+#include "shared/helpers.h"
 
 struct test_context;
 

tests/weston-test.c

@@ -40,6 +40,8 @@
 #include "src/weston-egl-ext.h"
 #endif /* ENABLE_EGL */
 
+#include "shared/helpers.h"
+
 struct weston_test {
 	struct weston_compositor *compositor;
 	struct weston_layer layer;