powerpc/mm: Move hpte_insert_repeating() prototype

It fixes this W=1 compile error :

../arch/powerpc/mm/book3s64/hash_utils.c:1867:6: error: no previous prototype for ‘hpte_insert_repeating’ [-Werror=missing-prototypes]
 1867 | long hpte_insert_repeating(unsigned long hash, unsigned long vpn,
      |      ^~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210104143206.695198-14-clg@kaod.org