gitignore: Ignore siteutils/buildbot_src/

To make git status and repo status more useful, ignore files that are
expected to exist on the production servers.

BUG=None
TEST=Manual check on real server.

Change-Id: I900250e5521d844091f18e2ca9a7b7978af294af
Reviewed-on: https://chromium-review.googlesource.com/233952
Tested-by: Don Garrett <dgarrett@chromium.org>
Reviewed-by: Prashanth B <beeps@chromium.org>
Commit-Queue: Don Garrett <dgarrett@chromium.org>
diff --git a/.gitignore b/.gitignore
index 42d0ae9..5393ca3 100644
--- a/.gitignore
+++ b/.gitignore
@@ -55,6 +55,7 @@
 site_utils/autotest_private/
 site_utils/autotest_tools/
 site_utils/devserver/
+site_utils/buildbot_src/
 
 # Pupplylab files
 puppylab/Vagrantfile