Moved helper macro to a discrete include file.

To help reduce code duplication and also 'kitchen-sink' includes
the ARRAY_LENGTH macro was moved to a stand-alone file and
referenced from the sources consuming it. Other macros will be
added in subsequent passes.

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>
diff --git a/clients/window.c b/clients/window.c
index 67afabd..c5e6f5a 100644
--- a/clients/window.c
+++ b/clients/window.c
@@ -67,6 +67,7 @@
 #include <linux/input.h>
 #include <wayland-client.h>
 #include "shared/cairo-util.h"
+#include "shared/helpers.h"
 #include "xdg-shell-client-protocol.h"
 #include "text-cursor-position-client-protocol.h"
 #include "workspaces-client-protocol.h"