Fix desktop-file-validate warnings. This is upstream commit:
https://trac.gajim.org/changeset/eaea5e65364b167d14dc191b00f114b4f5ccb508/

--- data/gajim.desktop.in.in.orig	2015-12-28 12:38:50 UTC
+++ data/gajim.desktop.in.in
@@ -3,7 +3,7 @@ Categories=Network;InstantMessaging;GTK;
 _Name=Gajim
 _GenericName=Jabber IM Client
 _Comment=A GTK+ Jabber client
-_Keywords=chat;messaging;im;jabber;xmpp;bonjour;voip
+_Keywords=chat;messaging;im;jabber;xmpp;bonjour;voip;
 Version=1.0
 Exec=gajim
 Icon=gajim