KVM: Move declaration of kvm_cpu_dirty_log_size() to kvm_dirty_ring.h

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 <[e-mail]>
Signed-off-by: Gavin Shan <[e-mail]>
Reviewed-by: Peter Xu <[e-mail]>
Signed-off-by: Marc Zyngier <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

arch/x86/include/asm/kvm_host.h

···
 #define GET_SMSTATE(type, buf, offset)		\
 	(*(type *)((buf) + (offset) - 0x7e00))
 
-int kvm_cpu_dirty_log_size(void);
-
 int memslot_rmap_alloc(struct kvm_memory_slot *slot, unsigned long npages);
 
 #define KVM_CLOCK_VALID_FLAGS						\

include/linux/kvm_dirty_ring.h

···
 
 #else /* CONFIG_HAVE_KVM_DIRTY_RING */
 
+int kvm_cpu_dirty_log_size(void);
 u32 kvm_dirty_ring_get_rsvd_entries(void);
 int kvm_dirty_ring_alloc(struct kvm_dirty_ring *ring, int index, u32 size);