[asio] Disable -Wreturn-std-move

This reports a single case in basic_resolver_results::begin() where we
should use std::move for efficiency, but we disable this warning for
since this is a third party code which would be ideally fixed in
upstream.

Change-Id: I840eee6db8206edd2c549d1a69ac64f94c2b3c31
diff --git a/BUILD.gn b/BUILD.gn
index 19eb0f3..4deaa91 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -673,7 +673,13 @@
     "asio/src/tests/unit/write_at.cpp",
   ]
 
-  cflags_cc = [ "-Wno-unused-variable" ]
+  cflags_cc = [
+    "-Wno-unused-variable",
+
+    # TODO(BLD-32): This reports a warning in basic_resolver_results::begin()
+    # where we should use std::move for efficiency.
+    "-Wno-return-std-move",
+  ]
 
   deps = [
     ":asio",