Fix unknown option '-msse2' warning

R=andrew@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/43169004

Cr-Commit-Position: refs/heads/master@{#9016}
diff --git a/webrtc/common_audio/BUILD.gn b/webrtc/common_audio/BUILD.gn
index 544a142..ec4a00d 100644
--- a/webrtc/common_audio/BUILD.gn
+++ b/webrtc/common_audio/BUILD.gn
@@ -195,7 +195,9 @@
       "resampler/sinc_resampler_sse.cc",
     ]
 
-    cflags = [ "-msse2" ]
+    if (is_posix) {
+      cflags = [ "-msse2" ]
+    }
 
     configs += [ "..:common_inherited_config" ]