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

Everything calls GetOSVersionName() instead, so no need to expose that.
Since it uses deprecated functions, this will make it easier to rewrite
it in a future change.

BUG=webrtc:6027
NOTRY=true

Review-Url: https://codereview.webrtc.org/2117823002
Cr-Commit-Position: refs/heads/master@{#13399}
diff --git a/webrtc/base/macutils.cc b/webrtc/base/macutils.cc
index 74b4919..72ca5cb 100644
--- a/webrtc/base/macutils.cc
+++ b/webrtc/base/macutils.cc
@@ -83,7 +83,7 @@
   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;