Fix header guard in thread_annotations.h.

It was using a non-WebRTC-named header guard, which could conflict with
other similarly named/intended headers.

BUG=None
NO_DEPENDENCY_CHECKS=true

Review-Url: https://codereview.webrtc.org/2548113002
Cr-Original-Commit-Position: refs/heads/master@{#15554}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 580df537ce829818448f0563c8aded6fc63b886b