nvram: Fix missing smp_lock.h in nvram

The bkl has been removed from nvram_llseek() and smp_lock.h was removed
because another patch in the same tree zapped the remaining usage of bkl
in the same file.  But this patch must have been excluded later, then we
still need the smp_lock.h headers for the bkl use in nvram_open().

This fixes the following build error:

  drivers/char/nvram.c: In function ‘nvram_open’:
  drivers/char/nvram.c:332: erreur: implicit declaration of function ‘lock_kernel’
  drivers/char/nvram.c:339: erreur: implicit declaration of function ‘unlock_kernel’
  make[2]: *** [drivers/char/nvram.o] Erreur 1
  make[1]: *** [drivers/char] Erreur 2
  make: *** [drivers] Erreur 2

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>