Fix build for go 1.9
Fix build for go 1.9

This change makes the #includes between credentials.go and remote.go
consistent to avoid a build error in go 1.9.

pks-t/git2go,clns/git2go,libgit2/git2go,clns/git2go,clns/git2go,libgit2/git2go,libgit2/git2go,pks-t/git2go