.mailmap: combine Stefan Beller's emails

Google mail has had the extension @googlemail.com for a long time
in Germany as @gmail.de was already taken by a competitor.
Nowadays the original gmail company isn't there anymore(?), hence
Googlemail also introduced @gmail.com in Germany, which I switched to.

This changed mail address of mine first appeared in 398dd4b
(2014-07-10, .mailmap: map different names with the same email
address together) ironically.

Signed-off-by: Stefan Beller <stefanbeller@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: stefanbeller

.mailmap

@@ -202,6 +202,7 @@ Seth Falcon <seth@userprimary.net> <sfalcon@fhcrc.org>
 Shawn O. Pearce <spearce@spearce.org>
 Simon Hausmann <hausmann@kde.org> <simon@lst.de>
 Simon Hausmann <hausmann@kde.org> <shausman@trolltech.com>
+Stefan Beller <stefanbeller@gmail.com> <stefanbeller@googlemail.com>
 Stefan Naewe <stefan.naewe@gmail.com> <stefan.naewe@atlas-elektronik.com>
 Stefan Naewe <stefan.naewe@gmail.com> <stefan.naewe@googlemail.com>
 Stefan Sperling <stsp@elego.de> <stsp@stsp.name>