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: refs/heads/master@{#11738}
diff --git a/.gitignore b/.gitignore
index 7bf3082..9215f12 100644
--- a/.gitignore
+++ b/.gitignore
@@ -150,6 +150,7 @@
 /tools/grit
 /tools/gyp
 /tools/isolate_driver.py
+/tools/mb
 /tools/memory
 /tools/protoc_wrapper
 /tools/python