Add RTC_EXPORT for NullSocketServer

NullSocketServer needs to be exported in order to use it in
JingleThreadWrapper in chromium.

Bug: none
Change-Id: I9bce49c764a1ca1c28fc44041d0d5f04f794066e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/173920
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#31174}
diff --git a/rtc_base/null_socket_server.h b/rtc_base/null_socket_server.h
index da22c79..7afa504 100644
--- a/rtc_base/null_socket_server.h
+++ b/rtc_base/null_socket_server.h
@@ -15,10 +15,11 @@
 #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;