.mailmap: add an alias for Tom Stellard

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/1999>
diff --git a/.mailmap b/.mailmap
index 94a05fb..2fa6af9 100644
--- a/.mailmap
+++ b/.mailmap
@@ -592,6 +592,7 @@
 
 Tom Stellard <thomas.stellard@amd.com> <tstellar@gmail.com>
 Tom Stellard <thomas.stellard@amd.com> Thomas Stellard <tom.stellard@amd.com>
+Tom Stellard <thomas.stellard@amd.com> <tstellar@redhat.com>
 
 Tomeu Vizoso <tomeu.vizoso@collabora.com> <tomeu@tomeuvizoso.net>