Add missing 'require_relative' in gitlab_net.rb
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.
```

yonh/gitlab-shell,gitlabhq/gitlab-shell,fscherwi/gitlab-shell,Jadaw1n/gitlab-shell,gitlabhq/gitlab-shell,dpursehouse/gitlab-shell,sonalkr132/gitlab-shell,bozaro/gitlab-shell,cui-liqiang/gitlab-shell,bozaro/gitlab-shell,fscherwi/gitlab-shell,dpursehouse/gitlab-shell,achako/gitlab-shell,gitlabhq/gitlab-shell,achako/gitlab-shell,yonh/gitlab-shell,dwrensha/gitlab-shell,cui-liqiang/gitlab-shell,Jadaw1n/gitlab-shell,sonalkr132/gitlab-shell,dwrensha/gitlab-shell,copystudy/gitlab-shell,copystudy/gitlab-shell