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: NPaul Mundt <lethal@linux-sh.org>

arch/sh/include/asm/pgtable.h

@@ -134,6 +134,7 @@ typedef pte_t *pte_addr_t;
 extern void pgtable_cache_init(void);
 
 struct vm_area_struct;
+struct mm_struct;
 
 extern void __update_cache(struct vm_area_struct *vma,
 			   unsigned long address, pte_t pte);