xen/x86: Remove the use of K&R functions

xen/x86: Remove the use of K&R functions

Clang-15 (as seen in the FreeBSD 14 tests) complains:

  arch/x86/time.c:1364:20: error: a function declaration without a
  prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
  s_time_t get_s_time()
                     ^
                      void

The error message is a bit confusing but appears to new as part of
-Wdeprecated-non-prototype which is part of supporting C2x which formally
removes K&R syntax.

Either way, fix the identified functions.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>