debug: optimize wp_log_level_is_enabled()

Add the 'const' attribute to let the compiler know that it doesn't
need to call it multiple times for the same debug level argument,
since the enabled log levels cannot change at runtime.

lib/wp/debug.h

@@ -20,7 +20,7 @@ G_BEGIN_DECLS
 #define WP_OBJECT_ARGS(object) G_OBJECT_TYPE_NAME(object), object
 
 WP_API
-gboolean wp_log_level_is_enabled (GLogLevelFlags log_level);
+gboolean wp_log_level_is_enabled (GLogLevelFlags log_level) G_GNUC_CONST;
 
 WP_API
 GLogWriterOutput wp_log_writer_default (GLogLevelFlags log_level,