`make install` installs `../include/userpci`

Add a way from a `Submakefile` for directories to be installed during
`make install`; use it to install `../include/userpci` to fix errors:

      install: cannot stat ‘../include/userpci/device.h’: No such file
      or directory
      [...]

fixes #298
Merge pull request #333 from zultron/fix-298

`make install` installs `../include/userpci`