gn: Fix cflags usage

R=brettw@chromium.org
TBR=kjellander@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/29519004

Patch from Cem Kocagil <ckocagil@chromium.org>.

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7198 4adac7df-926f-26a2-2b94-8c16560cd09d
diff --git a/webrtc/common_audio/BUILD.gn b/webrtc/common_audio/BUILD.gn
index f9bbd6a..036daa1 100644
--- a/webrtc/common_audio/BUILD.gn
+++ b/webrtc/common_audio/BUILD.gn
@@ -130,7 +130,7 @@
   }
 
   if (is_win) {
-    cflags += [
+    cflags = [
       "/wd4334",  # Ignore warning on shift operator promotion.
     ]
   }