make.sh tgz should handle windows binary

This allows `make tgz` to compress windows binary and `make all` to successfully run. Currently it fails with this message:
```
---> Making bundle: tgz (in bundles/1.4.0-dev/tgz)
...
cp: cannot stat '/go/src/github.com/docker/docker/bundles/1.4.0-dev/tgz/../cross/windows/386/docker-1.4.0-dev': No such file or directory
```

The windows exec binary is still being compressed with destination is `/usr/local/bin/docker` which is not too great, but i'm not sure how to fix it.
I don't think it makes a lot of sense there, no, but I think it's probably
OK for now.  I guess it depends on what MinGW / msysGit sets up, because
that's our current target environment.