Disable -Wtautological-unsigned-zero-compare

This raises an error in upstream version of Clang.

Change-Id: I41e26a1bf816a125e3b4fc16526060bd85a342cb
diff --git a/BUILD.gn b/BUILD.gn
index fd39331..66f6f47 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -13,6 +13,7 @@
     "-Wall",
     "-Wextra",
     "-Wno-missing-field-initializers",
+    "-Wno-tautological-unsigned-zero-compare",
     "-Wno-unused-parameter",
   ]
 }