Fix cpplint errors in p2p/

Bug: webrtc:5273
Change-Id: Ida3c2ca1bbb611026f34d7144db9ec24840bf7fb
Reviewed-on: https://webrtc-review.googlesource.com/26444
Commit-Queue: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21133}

p2p/base/asyncstuntcpsocket_unittest.cc

@@ -8,7 +8,9 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
+#include <list>
 #include <memory>
+#include <string>
 
 #include "p2p/base/asyncstuntcpsocket.h"
 #include "rtc_base/asyncsocket.h"