Disable -Wenum-compare-switch warning

This is a new warning type in Clang which causes build breakages in
icu, so disable it for now.

Change-Id: Ibfa6a98af0223b8e02eb08c6a8697adaa00a5731
diff --git a/BUILD.gn b/BUILD.gn
index 512ac2c..cdb08af 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -25,6 +25,9 @@
     # NaCl-related builds also fail looking for dlfcn.h when it's enabled.
     "U_ENABLE_DYLOAD=0",
   ]
+  cflags = [
+    "-Wno-enum-compare-switch",
+  ]
 
   is_component_build = false
   if (!is_component_build) {