.mailmap: add an alias for Thierry Reding

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/1999>
diff --git a/.mailmap b/.mailmap
index 9620ed3..56764bc 100644
--- a/.mailmap
+++ b/.mailmap
@@ -562,6 +562,7 @@
 
 Thierry Reding <treding@nvidia.com> <thierry@gilfi.de>
 Thierry Reding <treding@nvidia.com> <thierry.reding@avionic-design.de>
+Thierry Reding <treding@nvidia.com> <thierry.reding@gmail.com>
 
 Thierry Vignaud <thierry.vignaud@gmail.com> <tvignaud@mandriva.com>