KVM: PPC: Book3s: Fix warning about xics_rm_h_xirr_x

This fixes "no previous prototype":

arch/powerpc/kvm/book3s_hv_rm_xics.c:482:15:
warning: no previous prototype for 'xics_rm_h_xirr_x' [-Wmissing-prototypes]

Reported by the kernel test robot.

Fixes: b22af904

 ("KVM: PPC: Book3s: Remove real mode interrupt controller hcalls handlers")
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220622055235.1139204-1-aik@ozlabs.ru