mailmap: add entry for Greg Kurz

I had stopped using gkurz@linux.vnet.ibm.com a while back already but this
email address was shutdown last June when I quit IBM.  It's about time to
map it to groug@kaod.org.
Signed-off-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/159724692879.76040.4938578139173154028.stgit@bahia.lanSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

.mailmap

@@ -104,6 +104,7 @@ Gerald Schaefer <gerald.schaefer@linux.ibm.com> <geraldsc@linux.vnet.ibm.com>
 Greg Kroah-Hartman <greg@echidna.(none)>
 Greg Kroah-Hartman <gregkh@suse.de>
 Greg Kroah-Hartman <greg@kroah.com>
+Greg Kurz <groug@kaod.org> <gkurz@linux.vnet.ibm.com>
 Gregory CLEMENT <gregory.clement@bootlin.com> <gregory.clement@free-electrons.com>
 Hanjun Guo <guohanjun@huawei.com> <hanjun.guo@linaro.org>
 Heiko Carstens <hca@linux.ibm.com> <h.carstens@de.ibm.com>