mm/nobootmem.c: mark function as static

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: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/nobootmem.c

@@ -334,7 +334,7 @@ void * __init __alloc_bootmem_node_nopanic(pg_data_t *pgdat, unsigned long size,
 	return ___alloc_bootmem_node_nopanic(pgdat, size, align, goal, 0);
 }
 
-void * __init ___alloc_bootmem_node(pg_data_t *pgdat, unsigned long size,
+static void * __init ___alloc_bootmem_node(pg_data_t *pgdat, unsigned long size,
 				    unsigned long align, unsigned long goal,
 				    unsigned long limit)
 {