Declear prototype for kvm_init_loongson_ipi in interrupt.h.

Fix warning:
arch/mips/kvm/loongson_ipi.c:190:6: warning: no previous prototype for ‘kvm_init_loongson_ipi’ [-Wmissing-prototypes]
  190 | void kvm_init_loongson_ipi(struct kvm *kvm)
      |      ^~~~~~~~~~~~~~~~~~~~~

Fixes: f21db309 ("KVM: MIPS: Add Loongson-3 Virtual IPI interrupt support")
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>