powerpc: Fix compiliation with hugetlbfs enabled

arch/powerpc/mm/hugetlbpage.c: In function 'reserve_hugetlb_gpages':
arch/powerpc/mm/hugetlbpage.c:312:2: error: implicit declaration of function 'parse_args'

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