xen/ubsan: Fix UB in type_descriptor declaration

xen/ubsan: Fix UB in type_descriptor declaration

struct type_descriptor is arranged with a NUL terminated string following the
kind/info fields.

The only reason this doesn't trip UBSAN detection itself (on more modern
compilers at least) is because struct type_descriptor is only referenced in
suppressed regions.

Switch the declaration to be a real flexible member.  No functional change.

Fixes: 00fcf4dd8eb4 ("xen/ubsan: Import ubsan implementation from Linux 4.13")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>