Add src/results to .gitignore

src/tools/run-swarmed.py uses src/results as the output directory by
default. We should avoid treating them as source files in the repository.

Change-Id: I69ee66aa7c65382d0068bbd949fa761e638a5dc1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1781925
Auto-Submit: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695404}

.gitignore

@@ -302,3 +302,6 @@ vs-chromium-project.txt
 
 # Ignore IntelliJ files.
 .idea/
+
+# Ignore the default results output directory for tools/run-swarmed.py
+/results