KVM: PPC: Book3S HV: Include prototypes

It fixes these W=1 compile errors :

 CC [M]  arch/powerpc/kvm/book3s_64_mmu_hv.o
../arch/powerpc/kvm/book3s_64_mmu_hv.c:879:5: error: no previous prototype for ‘kvm_unmap_hva_range_hv’ [-Werror=missing-prototypes]
  879 | int kvm_unmap_hva_range_hv(struct kvm *kvm, unsigned long start, unsigned long end)
      |     ^~~~~~~~~~~~~~~~~~~~~~
../arch/powerpc/kvm/book3s_64_mmu_hv.c:888:6: error: no previous prototype for ‘kvmppc_core_flush_memslot_hv’ [-Werror=missing-prototypes]
  888 | void kvmppc_core_flush_memslot_hv(struct kvm *kvm,
      |      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
../arch/powerpc/kvm/book3s_64_mmu_hv.c:970:5: error: no previous prototype for ‘kvm_age_hva_hv’ [-Werror=missing-prototypes]
  970 | int kvm_age_hva_hv(struct kvm *kvm, unsigned long start, unsigned long end)
      |     ^~~~~~~~~~~~~~
../arch/powerpc/kvm/book3s_64_mmu_hv.c:1011:5: error: no previous prototype for ‘kvm_test_age_hva_hv’ [-Werror=missing-prototypes]
 1011 | int kvm_test_age_hva_hv(struct kvm *kvm, unsigned long hva)
      |     ^~~~~~~~~~~~~~~~~~~
../arch/powerpc/kvm/book3s_64_mmu_hv.c:1019:6: error: no previous prototype for ‘kvm_set_spte_hva_hv’ [-Werror=missing-prototypes]
 1019 | void kvm_set_spte_hva_hv(struct kvm *kvm, unsigned long hva, pte_t pte)
      |      ^~~~~~~~~~~~~~~~~~~

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210104143206.695198-20-clg@kaod.org