metag: fix mm/hugetlb.c build breakage

Commit 106c992a

 ("mm/hugetlb: add more arch-defined huge_pte
functions") 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'
  ...

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