Make muc_nick return a const char* const

In the documentation of the `muc_nick` function we can read:

> The nickname is owned by the chat room and should not be modified or freed

We should reflect that in the return type and `strdup` the value for the
plugin API.

Fixes: https://github.com/profanity-im/profanity/issues/2013

src/xmpp/muc.h

@@ -94,7 +94,7 @@ GList* muc_rooms(void);
 
 void muc_set_features(const char* const room, GSList* features);
 
-char* muc_nick(const char* const room);
+const char* const 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);