tools/libxenguest: fix build in stubdom environment

With introduction of the new byteswap infrastructure the build of
libxenguest for stubdoms was broken. Fix that again.

Fixes: 60dcff871e34 ("xen/decompressors: Remove use of *_to_cpup() helpers")
Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jason Andryuk <jason.andryuk@amd.com>
Acked-by: Anthony PERARD <anthony.perard@vates.tech>