libxl: avoid build warning when _libxl_types.h does not initially exist.

libxl: avoid build warning when _libxl_types.h does not initially exist.

Olaf Hering reports:
	if ! cmp _libxl_paths.h.2.tmp _libxl_paths.h; then mv -f _libxl_paths.h.2.tmp _libxl_paths.h; fi
	cmp: _libxl_paths.h: No such file or directory

Use "cmp -s" to silence the error. cmp returns 2 in this case and so the mv
does occur.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Tested-by: Olaf Hering <olaf@aepfle.de>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>