Fix format-security warnings
This patch should fix the FTBFS report in Debian sid (bug 997162): the format
string was omitted when displaying the message prefix and the mcabber version.
[Desktop Entry]
Encoding=UTF-8
Type=Application
Name=MCabber
GenericName=Jabber IM Client
Comment=Console Jabber Client
Exec=mcabber
Terminal=true
Categories=ConsoleOnly;System;Network;InstantMessaging;Chat;