Add missing is_user_authorized() native (#840)

* Add missing is_user_authorized() native

It is implemented in core, but missing in amxmodx.inc

* Fix typo

plugins/include/amxmodx.inc

@@ -921,6 +921,19 @@ native is_user_bot(index);
  */
 native is_user_hltv(index);
 
+/**
+ * Returns if the client is authorized.
+ *
+ * @note This does not throw an error if the provided index is out of the
+ *       1 to MaxClients range. That means you can safely use this native
+ *       without manually verifying that the index is a valid client index.
+ *
+ * @param index     Client index
+ *
+ * @return          1 if client is authorized, 0 otherwise
+ */
+native is_user_authorized(index);
+
 /**
  * Returns if the client is connected.
  *