metag: fix mm/hugetlb.c build breakage

metag: fix mm/hugetlb.c build breakage

Commit 106c992a5ebef ("mm/hugetlb: add more arch-defined huge_pte
functions") merged in v3.10-rc1.

The above commit added an include of <asm-generic/hugetlb.h> to each
architecture's <asm/hugetlb.h> (except s390).  Unfortunately metag was
missed which resulted in build errors when hugetlbfs is enabled (see
below).  Add the include for metag too to fix the build errors.

mm/hugetlb.c In function 'make_huge_pte':
mm/hugetlb.c +2250 : error: implicit declaration of function 'huge_pte_mkwrite'
mm/hugetlb.c +2250 : error: implicit declaration of function 'huge_pte_mkdirty'
mm/hugetlb.c +2250 : error: implicit declaration of function 'mk_huge_pte'
mm/hugetlb.c +2251 : error: incompatible types in assignment
mm/hugetlb.c +2254 : error: incompatible type for argument 1 of 'huge_pte_wrprotect'
mm/hugetlb.c In function 'set_huge_ptep_writable':
mm/hugetlb.c +2268 : error: incompatible types in assignment
mm/hugetlb.c In function '__unmap_hugepage_range':
mm/hugetlb.c +2383 : error: implicit declaration of function 'huge_pte_clear'
mm/hugetlb.c +2407 : error: implicit declaration of function 'huge_pte_dirty'
mm/hugetlb.c In function 'hugetlb_fault':
mm/hugetlb.c +2860 : error: implicit declaration of function 'huge_pte_write'
mm/hugetlb.c +2895 : error: incompatible types in assignment
mm/hugetlb.c In function 'hugetlb_change_protection':
mm/hugetlb.c +3047 : error: implicit declaration of function 'huge_pte_modify'
mm/hugetlb.c +3047 : error: incompatible type for argument 1 of 'pte_mkhuge'
make[1]: *** [mm/hugetlb.o] Error 1
make: *** [mm/hugetlb.o] Error 2

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Cc: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>