powerpc/mm: Fix build error with hugetlfs disabled

arch/powerpc/mm/slice.c:704:5: error: expected identifier or ‘(’ before numeric constant
 int is_hugepage_only_range(struct mm_struct *mm, unsigned long addr,
     ^
make[1]: *** [arch/powerpc/mm/slice.o] Error 1
make: *** [arch/powerpc/mm/slice.o] Error 2

This got introduced via 1217d34b
"powerpc: Ensure global functions include their prototype". We
started including linux/hugetlb.h with that patch and now we have

 #define is_hugepage_only_range(mm, addr, len)	0

with hugetlbfs disabled.

Fixes: 1217d34b

 ("powerpc: Ensure global functions include their prototype")
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>