.mailmap: add an alias for Neil Roberts

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/1999>
diff --git a/.mailmap b/.mailmap
index 340c534..f245633 100644
--- a/.mailmap
+++ b/.mailmap
@@ -435,6 +435,8 @@
 
 Neha Bhende <bhenden@vmware.com> <nbhende@vmware.com>
 
+Neil Roberts <nroberts@igalia.com> <neil@linux.intel.com>
+
 Nian Wu <nian.wu@intel.com> <nian@graphics.(none)>
 Nian Wu <nian.wu@intel.com> <nian@tinderbox.sh.intel.com>