sh: Fix up breakage from asm-generic/pgtable.h changes.

We require a forward declaration for mm_struct:

In file included from arch/sh/include/asm/pgtable.h:163,
                 from arch/sh/include/asm/io.h:21,
                 from arch/sh/kernel/machvec.c:20:
include/asm-generic/pgtable.h:104: error: 'struct mm_struct' declared inside parameter list
include/asm-generic/pgtable.h: In function 'ptep_get_and_clear_full':
include/asm-generic/pgtable.h:107: error: passing argument 1 of 'ptep_get_and_clear' from incompatible pointer type
include/asm-generic/pgtable.h:70: note: expected 'struct mm_struct *' but argument is of type 'struct mm_struct *'

Signed-off-by: Paul Mundt <lethal@linux-sh.org>