cleanup: Make muc_nick() return const char*

Fix `type qualifiers ignored on function return type`.

src/xmpp/muc.c

@@ -427,7 +427,7 @@ muc_rooms(void)
  * Return current users nickname for the specified room
  * The nickname is owned by the chat room and should not be modified or freed
  */
-const char* const
+const char*
 muc_nick(const char* const room)
 {
     ChatRoom* chat_room = g_hash_table_lookup(rooms, room);

src/xmpp/muc.h

@@ -93,7 +93,7 @@ GList* muc_rooms(void);
 
 void muc_set_features(const char* const room, GSList* features);
 
-const char* const muc_nick(const char* const room);
+const char* muc_nick(const char* const room);
 char* muc_password(const char* const room);
 
 void muc_nick_change_start(const char* const room, const char* const new_nick);