.mailmap: add an alias for Igor Gnatenko

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/1999>
diff --git a/.mailmap b/.mailmap
index d481f76..dd741ef 100644
--- a/.mailmap
+++ b/.mailmap
@@ -250,6 +250,8 @@
 Ian Romanick <ian.d.romanick@intel.com> <idr@freedesktop.org>
 Ian Romanick <ian.d.romanick@intel.com> <idr@us.ibm.com>
 
+Igor Gnatenko <i.gnatenko.brain@gmail.com> <ignatenko@redhat.com>
+
 Jakob Bornecrantz <wallbraker@gmail.com> <jakob@vmware.com>
 Jakob Bornecrantz <wallbraker@gmail.com> <jakob@aurora.(none)>
 Jakob Bornecrantz <wallbraker@gmail.com> <jakob@aurora.walkyrie.se>