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>

desktop-shell/exposay.c

@@ -28,6 +28,7 @@
 #include <linux/input.h>
 
 #include "shell.h"
+#include "shared/helpers.h"
 
 struct exposay_surface {
 	struct desktop_shell *shell;

desktop-shell/input-panel.c

@@ -32,6 +32,7 @@
 #include "shell.h"
 #include "desktop-shell-server-protocol.h"
 #include "input-method-server-protocol.h"
+#include "shared/helpers.h"
 
 struct input_panel_surface {
 	struct wl_resource *resource;