Fix various W=1 warnings in booke.c:

  arch/powerpc/kvm/booke.c:1008:5: error: no previous prototype for ‘kvmppc_handle_exit’ [-Werror=missing-prototypes]
   1008 | int kvmppc_handle_exit(struct kvm_vcpu *vcpu, unsigned int exit_nr)
        |     ^~~~~~~~~~~~~~~~~~
  arch/powerpc/kvm/booke.c:1009: warning: Function parameter or member 'vcpu' not described in 'kvmppc_handle_exit'
  arch/powerpc/kvm/booke.c:1009: warning: Function parameter or member 'exit_nr' not described in 'kvmppc_handle_exit'
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/r/202304020827.3LEZ86WB-lkp@intel.com/Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230403045314.3095410-1-mpe@ellerman.id.au