Fix SSH2 warnings

This PR fixes 2 warnings I've found in ```Git::Raw```.

1. The signature of ```git_cred_ssh_interactive_callback``` isn't quite right
2. ```libssh2.h``` should be included before ```git2.h``` to use ```libssh2```'s structures
Ah right, the same thing the user needs to do. I wonder why I don't see this with my compiler.