Ignore conflicts in db/schema.rb in Gitlab::EeCompatCheck

What does this MR do?

This makes the ee_compat_check ignore conflicts detected in db/schema.rb as they're easy enough to solve.

Why was this MR needed?

That was pointed out in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16578#note_56786612. /cc @_stark

Does this MR meet the acceptance criteria?

• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together