mac: Make GetOSVersion() private to a cc file.

webrtc/base/macutils.cc

Index: webrtc/base/macutils.cc
diff --git a/webrtc/base/macutils.cc b/webrtc/base/macutils.cc
index 74b49199647eea2bddc1ea9cd919f142aad5abea..72ca5cb3498c6eaaf09acc72b68c822fcc691c29 100644
--- a/webrtc/base/macutils.cc
+++ b/webrtc/base/macutils.cc
@@ -83,7 +83,7 @@ static bool GetGestalt(OSType ostype, int* value) {
   return false;
 }
 
-bool GetOSVersion(int* major, int* minor, int* bugfix) {
+static bool GetOSVersion(int* major, int* minor, int* bugfix) {
   ASSERT(major && minor && bugfix);
   if (!GetGestalt(gestaltSystemVersion, major)) {
     return false;