Fix header guard in thread_annotations.h.
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-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#15554}

TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc