Move the external decls for the C stubs into the xen/ and xenctrl/ libs

At the moment these files are the same but they may diverge. The xen/
version is tied to the mini-os APIs while the xenctrl/ version is tied
to the <xenctrl.h> APIs.

This allows the main package to be loaded in utop properly.

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