git: add skps to .gitignore

NOTRY=true

Review URL: https://codereview.chromium.org/924163003
diff --git a/.gitignore b/.gitignore
index 532a2cc..44d423f 100644
--- a/.gitignore
+++ b/.gitignore
@@ -14,6 +14,7 @@
 out
 platform_tools/chromeos/third_party/externals
 platform_tools/chromeos/toolchain
+skps
 third_party/externals
 tools/skp/page_sets/data/*.json
 tools/skp/page_sets/data/*.wpr