xen: change xen/[gntdev/gntalloc] to default m

When CONFIG_XEN is enabled the gntdev and gntalloc driver will be
compiled as a module by default.

[v2: Added the fix for the gntalloc driver as well]

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>