openrisc: add forward declaration for struct vm_area_struct

After removing linux/vmalloc.h from asm-generic/io.h, the following
warning occurs on openrisc:

   In file included from arch/openrisc/include/asm/io.h:33:0,
                    from include/linux/io.h:25,
                    from drivers/tty/serial/earlycon.c:19:
   arch/openrisc/include/asm/pgtable.h:424:2: warning: 'struct vm_area_struct' declared inside parameter list
     unsigned long address, pte_t *pte)
     ^
   arch/openrisc/include/asm/pgtable.h:424:2: warning: its scope is only this definition or declaration, which is probably not what you want

Reported-by: kbuild test robot <lkp@intel.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Stafford Horne <shorne@gmail.com>