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/calibrator.c b/clients/calibrator.c
index c5ecbe9..04c1cfc 100644
--- a/clients/calibrator.c
+++ b/clients/calibrator.c
@@ -35,10 +35,9 @@
 #include <wayland-client.h>
 
 #include "window.h"
+#include "shared/helpers.h"
 #include "shared/matrix.h"
 
-#define ARRAY_LENGTH(a) (sizeof (a) / sizeof (a)[0])
-
 /* Our points for the calibration must be not be on a line */
 static const struct {
 	float x_ratio, y_ratio;