powerpc/pmem: Include pmem prototypes

powerpc/pmem: Include pmem prototypes

It fixes this W=1 compile error :

../arch/powerpc/lib/pmem.c:51:6: error: no previous prototype for ‘arch_wb_cache_pmem’ [-Werror=missing-prototypes]
   51 | void arch_wb_cache_pmem(void *addr, size_t size)
      |      ^~~~~~~~~~~~~~~~~~
../arch/powerpc/lib/pmem.c:58:6: error: no previous prototype for ‘arch_invalidate_pmem’ [-Werror=missing-prototypes]
   58 | void arch_invalidate_pmem(void *addr, size_t size)
      |      ^~~~~~~~~~~~~~~~~~~~

Fixes: 32ce3862af3c ("powerpc/lib: Implement PMEM API")
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210104143206.695198-6-clg@kaod.org