.mailmap: add an alias for Philipp Zabel

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/1999>
diff --git a/.mailmap b/.mailmap
index 7a071bf..9e0c1af 100644
--- a/.mailmap
+++ b/.mailmap
@@ -478,6 +478,8 @@
 
 Peter Hutterer <peter.hutterer@who-t.net> <peter@cs.unisa.edu.au>
 
+Philipp Zabel <p.zabel@pengutronix.de> <philipp.zabel@gmail.com>
+
 Pierre-Eric Pelloux-Prayer <pelloux@gmail.com> pepp <pelloux@gmail.com>
 
 Pierre Willenbrock <pierre@pirsoft.de> Pierre Willenbrok <pierre@pirsoft.de>