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/presentation-shm.c b/clients/presentation-shm.c
index e81f5f4..cf4517c 100644
--- a/clients/presentation-shm.c
+++ b/clients/presentation-shm.c
@@ -36,11 +36,10 @@
 #include <time.h>
 
 #include <wayland-client.h>
+#include "shared/helpers.h"
 #include "shared/os-compatibility.h"
 #include "presentation_timing-client-protocol.h"
 
-#define ARRAY_LENGTH(a) (sizeof (a) / sizeof (a)[0])
-
 enum run_mode {
 	RUN_MODE_FEEDBACK,
 	RUN_MODE_FEEDBACK_IDLE,