powerpc/8xx: Fix build with hugetlbfs enabled

8xx uses the slice code when hugetlbfs is enabled. We missed a header
include on 8xx which resulted in the below build failure:

  config: mpc885_ads_defconfig + CONFIG_HUGETLBFS

  arch/powerpc/mm/slice.c: In function 'slice_get_unmapped_area':
  arch/powerpc/mm/slice.c:655:2: error: implicit declaration of function 'need_extra_context'
  arch/powerpc/mm/slice.c:656:3: error: implicit declaration of function 'alloc_extended_context'

on PPC64 the mmu_context.h was included via linux/pkeys.h
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/slice.c

@@ -35,6 +35,7 @@
 #include <asm/mmu.h>
 #include <asm/copro.h>
 #include <asm/hugetlb.h>
+#include <asm/mmu_context.h>
 
 static DEFINE_SPINLOCK(slice_convert_lock);