cogl-object-private.h: Include cogl-debug.h

If COGL_OBJECT_DEBUG is defined then cogl-object-private.h will call
COGL_NOTE in the ref and unref macros. For this to work the debug
header needs to also be included or COGL_NOTE won't necessarily be
defined.

clutter/cogl/cogl/cogl-object-private.h

@@ -30,6 +30,7 @@
 
 #include "cogl-types.h"
 #include "cogl-object.h"
+#include "cogl-debug.h"
 
 /* For compatability until all components have been converted */
 typedef struct _CoglObjectClass CoglHandleClass;