Add RTC_EXPORT for NullSocketServer
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 <2c708eb0a58047bf403399f50b3da33ffd2dd444@webrtc.org>
Reviewed-by: Niels Moller <0057859e47d61d90529d0f66d9ef692cefedb46a@webrtc.org>
Commit-Queue: Sergey Ulanov <8a30473e5cf3b93eac32943832fa7a9609291d70@chromium.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#31174}

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