.gitignore Win32's miniperl object files

Comments from the committer:
win32/mini/*.obj (and hence win32/mini/ since it contains nothing other than
*.obj) should be .gitignored by the top-level .gitignore file anyway, but
git clean -dfX doesn't remove them without this patch. (git clean -dfx does,
but some developers want to keep untracked files other than build products
explicitly listed by .gitignore files.)