Move the -Wno-unused-variable flag to a separate config

Change-Id: I1627e3e51fa98150b1aee3b558f44a73eace834a

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index e85c802..b542b3e 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -12,10 +12,13 @@
     ".",
     "include/",
   ]
-  cflags = [ "-Wno-unused-variable" ]
   defines = [ "GRPC_USE_PROTO_LITE" ]
 }
 
+config("grpc_warnings") {
+  cflags = [ "-Wno-unused-variable" ]
+}
+
 source_set("gpr") {
   sources = [
     "include/grpc/impl/codegen/atm.h",
@@ -797,6 +800,7 @@
     "//third_party/boringssl",
     "//third_party/protobuf:protobuf_lite",
   ]
+  configs += [ ":grpc_warnings" ]
   public_configs = [ ":grpc_config" ]
 }