Add missing 'require_relative' in gitlab_net.rb

Without this 'require' statement, you get the following error:

```
$ git ls-remote origin
/home/git/gitlab-shell/lib/gitlab_net.rb:30:in `check_access':
uninitialized constant GitlabNet::GitAccessStatus (NameError)
  from /home/git/gitlab-shell/lib/gitlab_shell.rb:63:in `validate_access'
  from /home/git/gitlab-shell/lib/gitlab_shell.rb:24:in `exec'
  from /home/git/gitlab-shell/bin/gitlab-shell:16:in `<main>'
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
```

lib/gitlab_net.rb

@@ -4,6 +4,7 @@ require 'json'
 
 require_relative 'gitlab_config'
 require_relative 'gitlab_logger'
+require_relative 'gitlab_access'
 
 class GitlabNet
   def check_access(cmd, repo, actor, changes)