Not all architectures like ARM64 need to override the function. Move
its declaration to kvm_dirty_ring.h to avoid the following compiling
warning on ARM64 when the feature is enabled.

  arch/arm64/kvm/../../../virt/kvm/dirty_ring.c:14:12:        \
  warning: no previous prototype for 'kvm_cpu_dirty_log_size' \
  [-Wmissing-prototypes]                                      \
  int __weak kvm_cpu_dirty_log_size(void)
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221110104914.31280-3-gshan@redhat.com