[PPC] Fix modpost warning

Mark pte_alloc_one_kernel as __init_refok to fix the following warning:

WARNING: arch/ppc/mm/built-in.o(.text+0x1114): Section mismatch: reference to .init.text:early_get_page (between 'pte_alloc_one_kernel' and 'v_mapped_by_tlbcam')

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