cogl-debug: Don't define _cogl_debug_flags when debugging disabled

_cogl_debug_flags isn't used when COGL_DEBUG is not defined so there's
no need to declare it.

Based on a patch by Fan, Chun-wei

http://bugzilla.clutter-project.org/show_bug.cgi?id=2561

cogl/cogl-debug.c

@@ -82,12 +82,8 @@ static const GDebugKey cogl_behavioural_debug_keys[] = {
 static const int n_cogl_behavioural_debug_keys =
   G_N_ELEMENTS (cogl_behavioural_debug_keys);
 
-#endif /* COGL_ENABLE_DEBUG */
-
 unsigned int _cogl_debug_flags[COGL_DEBUG_N_INTS];
 
-#ifdef COGL_ENABLE_DEBUG
-
 static void
 _cogl_parse_debug_string_for_keys (const char *value,
                                    gboolean enable,