powerpc/mm/64s: Fix no previous prototype warning

powerpc/mm/64s: Fix no previous prototype warning

As reported by lkp:

  arch/powerpc/mm/book3s64/radix_tlb.c:646:6: warning: no previous
  prototype for function 'exit_lazy_flush_tlb'

Fix it by moving the prototype into the existing header.

Fixes: 032b7f08932c ("powerpc/64s/radix: serialize_against_pte_lookup IPIs trim mm_cpumask")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210210130804.3190952-2-mpe@ellerman.id.au