Include git2/odb_backend

Required to avoid compilation barfing with:

```
src/repository.c:417:11: error: incomplete definition of type 'struct git_odb_stream'    stream->write(stream, buffer, buflen);
    ~~~~~~^/usr/local/include/git2/types.h:90:16: note: forward declaration of 'struct git_odb_stream'typedef struct git_odb_stream git_odb_stream;               ^
src/repository.c:418:17: error: incomplete definition of type 'struct git_odb_stream'    err = stream->finalize_write(&oid, stream);          ~~~~~~^/usr/local/include/git2/types.h:90:16: note: forward declaration of 'struct git_odb_stream'typedef struct git_odb_stream git_odb_stream;               ^src/repository.c:419:11: error: incomplete definition of type 'struct git_odb_stream'    stream->free(stream);    ~~~~~~^
```