mkosi: fix suffix for roothash file
mkosi: fix suffix for roothash file

The file is supposed to be called .usrhash if we do a build without a
root dir. At least that's what systemd's automatic discovery looks for,
see:

https://www.freedesktop.org/software/systemd/man/systemd.exec.html#RootHash=

systemd/mkosi,systemd/mkosi