Fix mempolicy.h build error

Fix mempolicy.h build error

<linux/mempolicy.h> uses struct mm_struct and relies on a definition or
declaration somehow magically being dragged in which may result in a
build:

  CC      mm/mempolicy.o
In file included from mm/mempolicy.c:69:
include/linux/mempolicy.h:150: warning: 'struct mm_struct' declared inside parameter list
include/linux/mempolicy.h:150: warning: its scope is only this definition or declaration, which is probably not what you want
include/linux/mempolicy.h:174: warning: 'struct mm_struct' declared inside parameter list
mm/mempolicy.c:673: error: conflicting types for 'do_migrate_pages'
include/linux/mempolicy.h:174: error: previous declaration of 'do_migrate_pages' was here
mm/mempolicy.c:1696: error: conflicting types for 'mpol_rebind_mm'
include/linux/mempolicy.h:150: error: previous declaration of 'mpol_rebind_mm' was here
make[1]: *** [mm/mempolicy.o] Error 1
make: *** [mm] Error 2
$

Including <linux/sched.h> is a step into direction of include hell so
fixed by adding a forward declaration of struct mm_struct instead.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Adrian Bunk <bunk@stusta.de>