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.lan
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>