gn: Disable new GCC redundant move warning

Disable the new GCC warning for a redundant move in "return
std::move(...)" because we want to maintain this pattern for
compatibility for older compilers.

Change-Id: I4560cf16245b54c1bd70855bb6348f0db41a01de
diff --git a/gn/standalone/BUILD.gn b/gn/standalone/BUILD.gn
index 9928a72..f11bb5f 100644
--- a/gn/standalone/BUILD.gn
+++ b/gn/standalone/BUILD.gn
@@ -54,6 +54,11 @@
       "-Wno-unknown-sanitizers",
     ]
   }
+
+  if (!is_clang) {
+    # Use return std::move(...) for compatibility with old compilers.
+    cflags += [ "-Wno-redundant-move" ]
+  }
 }
 
 config("no_exceptions") {