xen: add wc_sec_hi to struct shared_info

Xen added this in 2015 (Xen 4.6). On x86_64 and Arm it fills what was
previously a 32-bit hole in the generic shared_info structure; on
i386 it had to go at the end of struct arch_shared_info.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>