Move thread_ conditional back under defines.
Move thread_ conditional back under defines.

Unbreaks Windows builds.

BUG=
TBR=tommi@webrtc.org

Review URL: https://codereview.webrtc.org/1476543002 .

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#10778}

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