irc: fix separator between nick and host in bar item "irc_nick_host"

Use "!" instead of "@" to separate nick from host, like it is done in IRC
protocol.

The nick and host is now displayed as "nick!user@host" instead of
"nick@user@host".

src/plugins/irc/irc-bar-item.c

@@ -472,7 +472,7 @@ irc_bar_item_nick_host (const void *pointer, void *data,
 
     snprintf (buf, sizeof (buf), "%s%s%s",
               server->nick,
-              (server->host) ? "@" : "",
+              (server->host) ? "!" : "",
               (server->host) ? server->host : "");
 
     return strdup (buf);