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 <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Signed-off-by: Tobias Klauser <[e-mail]>
Signed-off-by: Stafford Horne <[e-mail]>

arch/openrisc/include/asm/pgtable.h

···
 
 extern pgd_t swapper_pg_dir[PTRS_PER_PGD]; /* defined in head.S */
 
+struct vm_area_struct;
+
 /*
  * or32 doesn't have any external MMU info: the kernel page
  * tables contain all the necessary information.