Fixes this:

  drivers/char/ramoops.c: In function 'ramoops_init':
  drivers/char/ramoops.c:221: error: implicit declaration of function 'IS_ERR'
  drivers/char/ramoops.c:222: error: implicit declaration of function 'PTR_ERR'

If it actually builds on other platforms, it's probably getting
linux/err.h via some other #include.
Signed-off-by: NJames Bottomley <JBottomley@Parallels.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>