jabber: Fix missing optional argument for the alert jabber-alert-echo function.

Jabber alert function should accept an optional second argument containing a
title of the message. The function mimic the default implementation of
jabber-alert-echo by showing the title if it is present and a message body
otherwise.

This commit fixes an error which is thrown when jabber receives a new notification:
(wrong-number-of-arguments (lambda (msg) (if (minibuffer-prompt) nil (message "%s" msg))) 2)

layers/+chat/jabber/funcs.el

@@ -15,6 +15,6 @@
   ;; Disable the minibuffer getting jabber messages when active
   ;; See http://www.emacswiki.org/JabberEl
   (define-jabber-alert echo "Show a message in the echo area"
-    (lambda (msg)
+    (lambda (msg &optional title)
       (unless (minibuffer-prompt)
-        (message "%s" msg)))))
+        (message "%s" (or title msg))))))