x86/vpmu: Fix build following vmfork addition

x86/vpmu: Fix build following vmfork addition

GCC with IBT extensions complains:

  arch/x86/cpu/vpmu.c:351:15: error: conflicting types for 'vpmu_save_force'; have 'void(void *)' with implied 'nocf_check' attribute
    351 | void cf_check vpmu_save_force(void *arg)
        |               ^~~~~~~~~~~~~~~
  In file included from ./arch/x86/include/asm/domain.h:10,
                   from ./include/xen/domain.h:8,
                   from ./include/xen/sched.h:11,
                   from ./include/xen/event.h:12,
                   from arch/x86/cpu/vpmu.c:23:
  ./arch/x86/include/asm/vpmu.h:117:6: note: previous declaration of 'vpmu_save_force' with type 'void(void *)'
    117 | void vpmu_save_force(void *arg);
        |      ^~~~~~~~~~~~~~~

Adjust the declaraion.

Fixes: 755087eb9b10 ("xen/mem_sharing: support forks with active vPMU state")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>