autotest: Add swarming_state to gitignore

BUG=chromium:844767
TEST=None

Change-Id: I8a1c27dee6cb669679db20027400ea1e2b70e83b
Reviewed-on: https://chromium-review.googlesource.com/1066484
Tested-by: Allen Li <ayatane@chromium.org>
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
diff --git a/.gitignore b/.gitignore
index ef18919..127af18 100644
--- a/.gitignore
+++ b/.gitignore
@@ -72,3 +72,6 @@
 
 # HWID key file used by site_utils/hwid_lib.py
 hwid_key
+
+# Present on Skylab drones
+/swarming_state/