.mailmap: add Stefan Bellers corporate mail address

Note that despite the private address being first and primary,
Google owns the copyright on this patch as any other patch I'll be
sending signed off by the sbeller@google.com address.

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

.mailmap

@@ -205,6 +205,7 @@ 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 Beller <stefanbeller@gmail.com> <sbeller@google.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>