`git2/sys/mempack.h` fails to include `git2/buffer.h`

Header file `git2/sys/mempack.h` should include `git2/buffer.h`:
~~~
/usr/local/include/git2/sys/mempack.h:67:22: error: unknown type name 'git_buf'
~~~

### Reproduction steps

### Expected behavior

### Actual behavior

### Version of libgit2 (release number or SHA1)
0.26.0

### Operating system(s) tested
