message: Add RTC_EXPORT for NullSocketServer
  NullSocketServer needs to be exported in order to use it in
  JingleThreadWrapper in chromium.
  Cr-Commit-Position: refs/heads/master@{#31174}
diff: #include "rtc_base/event.h"
  #include "rtc_base/socket.h"
  #include "rtc_base/socket_server.h"
  +#include "rtc_base/system/rtc_export.h"
  namespace rtc {
  -class NullSocketServer : public SocketServer {
  +class RTC_EXPORT NullSocketServer : public SocketServer {
  public:
  NullSocketServer();
  ~NullSocketServer() override;
  