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 1217d34
"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: 1217d34 ("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>

arch/powerpc/mm/slice.c

@@ -682,6 +682,7 @@ void slice_set_range_psize(struct mm_struct *mm, unsigned long start,
 	slice_convert(mm, mask, psize);
 }
 
+#ifdef CONFIG_HUGETLB_PAGE
 /*
  * is_hugepage_only_range() is used by generic code to verify whether
  * a normal mmap mapping (non hugetlbfs) is valid on a given area.
@@ -726,4 +727,4 @@ int is_hugepage_only_range(struct mm_struct *mm, unsigned long addr,
 #endif
 	return !slice_check_fit(mask, available);
 }
-
+#endif