Add strchrnul.

Bug: http://b/18374026
Change-Id: Iea923309c090a51a2d41c5a83320ab3789f40f1c
diff --git a/libc/Android.mk b/libc/Android.mk
index ca20d3d..437f8b2 100644
--- a/libc/Android.mk
+++ b/libc/Android.mk
@@ -214,6 +214,7 @@
     bionic/socket.cpp \
     bionic/stat.cpp \
     bionic/statvfs.cpp \
+    bionic/strchrnul.cpp \
     bionic/strerror.cpp \
     bionic/strerror_r.cpp \
     bionic/strsignal.cpp \