make getFD() a const method on AsyncUDPServerSocket

Summary: no need to be mutable, since it only calls AsyncUDPSocket::getFD() which is a const method

Test Plan: just compiled -- simple change

Reviewed By: afrind@fb.com

Subscribers: trunkagent, doug, folly-diffs@

FB internal diff: D1802765

Signature: t1:1802765:1422307972:eab6ab4b16407df6e5034cef78c79f45cd9fa46f

folly/io/async/AsyncUDPServerSocket.h

@@ -114,7 +114,7 @@ class AsyncUDPServerSocket : private AsyncUDPSocket::ReadCallback {
     socket_->resumeRead(this);
   }
 
-  int getFD() {
+  int getFD() const {
     CHECK(socket_) << "Need to bind before getting FD";
     return socket_->getFD();
   }