gitignore: add in hwid_key to .gitignore file.

BUG=chromium:571087
TEST=None

Change-Id: Ic8cf60b2381c4e85368100230b47b809735c8620
Reviewed-on: https://chromium-review.googlesource.com/329197
Commit-Ready: Kevin Cheng <kevcheng@chromium.org>
Tested-by: Kevin Cheng <kevcheng@chromium.org>
Reviewed-by: Dan Shi <dshi@google.com>
diff --git a/.gitignore b/.gitignore
index d09e866..4dd91a2 100644
--- a/.gitignore
+++ b/.gitignore
@@ -72,3 +72,6 @@
 
 # Shadow config for perf dashboard
 tko/perf_upload/perf_dashboard_shadow_config.json
+
+# HWID key file used by site_utils/hwid_lib.py
+hwid_key