tools/libxentoolcore: Fill in LIBHEADERS

tools/libxentoolcore: Fill in LIBHEADERS

c/s 4664034cd replaced a glob over include/*.h with an expectation that
LIBHEADER was suitably set for libraries which didn't have a single,
consistently named, header file.

This wasn't true for xentoolcore, which lost xentoolcore_internal.h as a
consequence, and failed an API/ABI check vs 4.14

Fixes: 4664034cd ("tools/libs: move official headers to common directory")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>