libs/light: Fix build, fix missing _libxl_types_json.h

libs/light: Fix build, fix missing _libxl_types_json.h

Make may not have copied "_libxl_types_json.h" into $(XEN_INCLUDE)
before starting to build the different objects.

Make sure that the generated headers are copied into $(XEN_INCLUDE)
before using them. This is achieved by telling make about which
headers are needed to use "libxl_internal.h" which use "libxl_json.h"
which uses "_libxl_types_json.h". "libxl_internal.h" also uses
"libxl.h" so add it to the list.

This also prevent `gcc` from using a potentially installed headers
from a previous version of Xen.

Reported-by: Per Bilse <per.bilse@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>