Include libgit2 headers in our module.
Get the module working

The swift compiler is confused by the user header search paths in the
git2/sys/* headers. This includes a second copy of the git2 headers so
that the swift compiler will find them when included from these
headers. It happens to still work with the modulemap.
Merge pull request #424 from libgit2/module

Include libgit2 headers in our module