MIPS: Add linux/mm_types.h to pgtable.h

With the addition of transparent huge pages, pgtable.h uses struct page.
However, it is possible to include pgtable.h without anything defining
struct page.  So add the include to get it.

Signed-off-by: Corey Minyard <cminyard@mvista.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>