mm: <asm-generic/pgtable.h> must include <linux/mm_types.h>

Commit e2cda3226481 ("thp: add pmd mangling generic functions") replaced
some macros in <asm-generic/pgtable.h> with inline functions.

If the functions are to be defined (not all architectures need them)
then struct vm_area_struct must be defined first. So include
<linux/mm_types.h>.

Fixes a build failure seen in Debian:

CC [M] drivers/media/dvb/mantis/mantis_pci.o
In file included from arch/arm/include/asm/pgtable.h:460,
from drivers/media/dvb/mantis/mantis_pci.c:25:
include/asm-generic/pgtable.h: In function 'ptep_test_and_clear_young':
include/asm-generic/pgtable.h:29: error: dereferencing pointer to incomplete type

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>