KVM: always declare prototype for kvm_arch_irqchip_in_kernel

Architecture code might want to use it even if CONFIG_HAVE_KVM_IRQ_ROUTING
is false; for example PPC XICS has KVM_IRQ_LINE and wants to use
kvm_arch_irqchip_in_kernel from there, but it does not have
KVM_SET_GSI_ROUTING so the prototype was not provided.

Fixes: d663b8a2

 ("KVM: replace direct irq.h inclusion")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>