.mailmap: add a couple of aliases for Steinar H. Gunderson

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/1999>
diff --git a/.mailmap b/.mailmap
index 96127c8..0baad02 100644
--- a/.mailmap
+++ b/.mailmap
@@ -546,6 +546,9 @@
 
 Stefan Brüns <stefan.bruens@rwth-aachen.de> <Stefan.Bruens@rwth-aachen.de>
 
+Steinar H. Gunderson <sgunderson@bigfoot.com> <sesse@google.com>
+Steinar H. Gunderson <sgunderson@bigfoot.com> <steinar+mesa@gunderson.no>
+
 Stéphane Marchesin <marcheu@chromium.org> Stephane Marchesin <marchesin@icps.u-strasbg.fr>
 Stéphane Marchesin <marcheu@chromium.org> Stephane Marchesin <stephane.marchesin@gmail.com>