Disable -Wenum-compare

This is triggering an error with the latest Clang.

Change-Id: I5419bf9dac963f4e75baaa2e601735f9bebfbb07
diff --git a/BUILD.gn b/BUILD.gn
index 09feabf..5b9ed24 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -48,6 +48,7 @@
     "-Wno-sign-compare",
     "-Wno-tautological-pointer-compare",
     "-Wno-unused-function",
+    "-Wno-enum-compare",
   ]
   defines = [
     "HAVE_CONFIG_H",