Add tools/mb to .gitignore
Add tools/mb to .gitignore

Should have been done in
https://codereview.webrtc.org/1730113002/

TBR=phoglund@webrtc.org

Review URL: https://codereview.webrtc.org/1727123003 .

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#11738}

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