.mailmap: map Clemens Buchacher's mail addresses

We have three email addresses for Clemens in our commit history, two of
them bouncing. Let's map the latter to the only one that still works.

Pointed out by Gábor Szeder.
Signed-off-by: NJohannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

.mailmap

@@ -40,6 +40,8 @@ Cord Seele <cowose@gmail.com> <cowose@googlemail.com>
 Christian Couder <chriscool@tuxfamily.org> <christian.couder@gmail.com>
 Christian Stimming <stimming@tuhh.de> <chs@ckiste.goetheallee>
 Christopher Díaz Riveros <chrisadr@gentoo.org> Christopher Diaz Riveros
+Clemens Buchacher <drizzd@gmx.net> <drizzd@aon.at>
+Clemens Buchacher <drizzd@gmx.net> <clemens.buchacher@intel.com>
 Csaba Henk <csaba@gluster.com> <csaba@lowlife.hu>
 Dan Johnson <computerdruid@gmail.com>
 Dana L. How <danahow@gmail.com> <how@deathvalley.cswitch.com>