Remove C stubs dependency from the parent package

- xen-gnt contains only the pure OCaml code, stubs are used by
  sub-packages
- the Gnt module has been forked in xen/ and xenctrl/ and the APIs
  used internally might diverge. xen/ follows mini-os while xenctrl/
  follows <xenctrl.h>
- the inheap implementation nolonger uses Io_page since it doesn't
  need true page-aligned buffers.

Signed-off-by: David Scott <dave.scott@citrix.com>