Mark function as static in nobootmem.c because it is not used outside
this file.

This eliminates the following warning in mm/nobootmem.c:

  mm/nobootmem.c:324:15: warning: no previous prototype for `___alloc_bootmem_node' [-Wmissing-prototypes]
Signed-off-by: NRashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>
Acked-by: NDavid Rientjes <rientjes@google.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>