Update .mailmap

Before:

$ git shortlog -n -s | grep Han
    14    Han-Wen Nienhuys
     1    Han-Wen NIenhuys

After:

$ git shortlog -n -s | grep Han
    15    Han-Wen Nienhuys

Change-Id: I2874713f120f09bcbd33a1c48645e25fb5bf9259
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>

.mailmap

@@ -1,3 +1,4 @@
+Han-Wen Nienhuys <hanwen@google.com>         Han-Wen NIenhuys <hanwen@google.com>
 Mark Ingram <markdingram@gmail.com>          markdingram <markdingram@gmail.com>
 Roberto Tyley <roberto.tyley@guardian.co.uk> roberto <roberto.tyley@guardian.co.uk>
 Saša Živkov <sasa.zivkov@sap.com>            Sasa Zivkov <sasa.zivkov@sap.com>