Add sdk/system-images to git ignore.

This allows a user or script to fetch system images
into the sdk via something like:
  $ android update sdk --no-ui --all --filter "system-image"

System images are necessary for starting an emulator.

Change-Id: I0fd01cb37264983803d4ca3ee19ed45817873a66
Reviewed-on: https://chromium-review.googlesource.com/264801
Reviewed-by: Chris Hopman <cjhopman@chromium.org>
Tested-by: Tony Gentilcore <tonyg@chromium.org>
Commit-Queue: Aurimas Liutikas <aurimas@chromium.org>