monitor: fix maybe-uninitialized

monitor: fix maybe-uninitialized

../monitor/misc.c: In function ‘mon_get_cpu_sync’:
/home/elmarco/src/qq/include/sysemu/hw_accel.h:22:9: error: ‘cpu’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
   22 |         kvm_cpu_synchronize_state(cpu);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../monitor/misc.c:397:15: note: ‘cpu’ was declared here
  397 |     CPUState *cpu;
      |               ^~~

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>