.mailmap: add an alias for Rob Clark

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/1999>
diff --git a/.mailmap b/.mailmap
index 409d9a0..2ecf0b9 100644
--- a/.mailmap
+++ b/.mailmap
@@ -507,6 +507,7 @@
 
 Rob Clark <robclark@freedesktop.org> <Rob Clark robdclark@freedesktop.org>
 Rob Clark <robclark@freedesktop.org> <robdclark@gmail.com>
+Rob Clark <robclark@freedesktop.org> <robdclark@chromium.org>
 
 Robert Bragg <robert@sixbynine.org> <robert@linux.intel.com>