fix: allow ignored directories that already end in a path separator

[jedevc]

Previously, a config of the form:

lint:
  directories:
    exclude:
      - vendor/

would not ignore files in the vendor directory, since we would be attempting to match paths that begin with vendor//.

To fix this, we only append the path separator when the target does not already end in one.

[yoheimuta]

I've just released it as https://github.com/yoheimuta/protolint/releases/tag/v0.45.1.