[POWERPC] Fix modpost warning

Mark pte_alloc_one_kernel as __init_refok to fix the following warning:

WARNING: arch/powerpc/mm/built-in.o(.text+0x1068): Section mismatch: reference to .init.text:early_get_page (between 'pte_alloc_one_kernel' and 'steal_context')

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>