Move ARRAY_LENGTH out of public headers

Exporting unprefixed symbols is a pretty bad idea so don't do that.
Instea of redefining it WL_ARRAY_LENGTH, we just move the define to
our private header.  The scanner generates code that uses ARRAY_LENGTH,
but we can just make it count the number elements and emit an integer
constant instead.

src/event-loop.c

@@ -35,6 +35,7 @@
 #include <unistd.h>
 #include <assert.h>
 #include "wayland-server.h"
+#include "wayland-private.h"
 #include "wayland-os.h"
 
 struct wl_event_loop {