Fix the include guard in transmit_mixer.h
Fix the include guard in transmit_mixer.h

R=henrika@webrtc.org, stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/5879004

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 0062a6d0995c24f423504aba83b3f086cabec924

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