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);
    ~~~~~~^
/usr/local/include/git2/types.h:90:16: note: forward declaration of 'struct git_odb_stream'
typedef struct git_odb_stream git_odb_stream;
               ^
3 errors generated.
error: command 'clang' failed with exit status 1

```