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/src/compositor.c b/src/compositor.c
index 2f7864f..2f89b39 100644
--- a/src/compositor.c
+++ b/src/compositor.c
@@ -61,6 +61,7 @@
 #include "compositor.h"
 #include "scaler-server-protocol.h"
 #include "presentation_timing-server-protocol.h"
+#include "shared/helpers.h"
 #include "shared/os-compatibility.h"
 #include "git-version.h"
 #include "version.h"