autoninja: Ignore build_telemetry.cfg

build_telemetry.cfg is generated by build_telemetry.

Bug: 345113094
Change-Id: I193098e641c55e06177bc6cfc6626cea678426b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5724919
Reviewed-by: Yiwei Zhang <yiwzhang@google.com>
Commit-Queue: Yiwei Zhang <yiwzhang@google.com>
Auto-Submit: Junji Watanabe <jwata@google.com>

.gitignore

@@ -86,6 +86,7 @@ testing_support/google_appengine
 
 # Ignore win/mac monitoring config. It is unique for each depot_tools checkout.
 /metrics.cfg
+/build_telemetry.cfg
 
 # Ignore git traces produced by git push on git-cl upload.
 /traces