.mailmap: add an alias for Colin McDonald

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/1999>
diff --git a/.mailmap b/.mailmap
index 81db771..65f0eec 100644
--- a/.mailmap
+++ b/.mailmap
@@ -146,6 +146,8 @@
 Claudio Ciccani <klan@directfb.org> <klan@users.sf.net>
 Claudio Ciccani <klan@directfb.org> <klan@users.sourceforge.net>
 
+Colin McDonald <cjmmail10-bz@yahoo.co.uk> <cjmcdonald@qinetiq.com>
+
 Connor Abbott <cwabbott0@gmail.com> <connor.w.abbott@intel.com>
 Connor Abbott <cwabbott0@gmail.com> <connor.abbott@intel.com>