Added warning suppression for -Wnull-pointer-airthmetic

Change-Id: Id529fc41f18ab0550b48344eface7d0c11eb36ec
diff --git a/BUILD.gn b/BUILD.gn
index 19b6492..729659b 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -134,6 +134,7 @@
       cflags = [
         # reg3264type in x86expr.c is unused.
         "-Wno-unused-local-typedef",
+        "-Wno-null-pointer-arithmetic",
       ]
     }
   }