task_nommu: fix compile failing bug because of spilt file.h

  CC      fs/proc/task_nommu.o
fs/proc/task_nommu.c: In function ‘task_mem’:
fs/proc/task_nommu.c:55: error: dereferencing pointer to incomplete type
make[2]: *** [fs/proc/task_nommu.o] Error 1
make[1]: *** [fs/proc] Error 2
make: *** [fs] Error 2

Signed-off-by: Bryan Wu <cooloney@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>