When CONFIG_PROC_FS is not set, proc_salinfo_show() is not used.  Mark the
function as __maybe_unused to quieten the warning message.

../arch/ia64/kernel/salinfo.c:584:12: warning: 'proc_salinfo_show' defined but not used [-Wunused-function]
  584 | static int proc_salinfo_show(struct seq_file *m, void *v)
      |            ^~~~~~~~~~~~~~~~~

Link: https://lkml.kernel.org/r/20230223034309.13375-1-rdunlap@infradead.org
Fixes: 3f3942ac ("proc: introduce proc_create_single{,_data}")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>