Fix check_mocks make target

What does this MR do?

Fixes the make check_mocks target.

Why was this MR needed?

Current scripting of check_mocks make target doesn't fail if there is a missing mock for a newly added interface. It's that because:

1. git ls-files doesn't list files not tracked by git. And even if it would, then
2. git diff would not show any change on a mock file freshly created by $(MAKE) mocks call.

Currently the check will not fail if there is a new interface for which a mock was not generated. An example job that should fail but didn't is https://gitlab.com/gitlab-org/gitlab-runner/-/jobs/714087838.

What's the best way to test this MR?

What are the relevant issue numbers?