Adjust function definitions in xen's control.c to avoid clang 15 warnings

Adjust function definitions in xen's control.c to avoid clang 15 warnings

With clang 15, the following -Werror warnings are produced:

    sys/dev/xen/control/control.c:188:15: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
    xctrl_poweroff()
                  ^
                   void
    sys/dev/xen/control/control.c:194:13: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
    xctrl_reboot()
                ^
                 void
    sys/dev/xen/control/control.c:207:14: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
    xctrl_suspend()
                 ^
                  void
    sys/dev/xen/control/control.c:344:12: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
    xctrl_crash()
               ^
                void

This is because xctrl_poweroff(), xctrl_reboot(), xctrl_suspend(), and
xctrl_crash() are declared with (void) argument lists, but defined with
empty argument lists. Make the definitions match the declarations.

MFC after:	3 days