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/xwayland/window-manager.c b/xwayland/window-manager.c
index 9b4e088..c3a57c1 100644
--- a/xwayland/window-manager.c
+++ b/xwayland/window-manager.c
@@ -42,6 +42,7 @@
 #include "cairo-util.h"
 #include "compositor.h"
 #include "hash.h"
+#include "shared/helpers.h"
 
 struct wm_size_hints {
 	uint32_t flags;