Stop relying on ARRAY_LENGTH being defined in wayland-util.h

Time to break a bad habit.
diff --git a/src/compositor.h b/src/compositor.h
index 4c61db7..19ab11e 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -37,6 +37,7 @@
 #include "../shared/config-parser.h"
 #include "weston-egl-ext.h"
 
+#define ARRAY_LENGTH(a) (sizeof (a) / sizeof (a)[0])
 
 struct weston_transform {
 	struct weston_matrix matrix;