frv: fix fallout from "remove sched.h from mm.h" patch

/home/rpjday/AMD/k/topics/0_hi/hi1.c:15: error: dereferencing pointer to incomplete type
/home/rpjday/AMD/k/topics/0_hi/hi1.c:16: error: dereferencing pointer to incomplete type
Signed-off-by: Alexey Dobriyan <adobriyan@sw.ru>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/asm-frv/pgtable.h

@@ -25,7 +25,7 @@
 #include <linux/slab.h>
 #include <linux/list.h>
 #include <linux/spinlock.h>
-struct mm_struct;
+#include <linux/sched.h>
 struct vm_area_struct;
 #endif