Input: hp_sdc_rtc - mark an unused function as __maybe_unused

Input: hp_sdc_rtc - mark an unused function as __maybe_unused

When CONFIG_PROC_FS is not set, one procfs-related function is not
used, causing a build error or warning.
Mark this function as __maybe_unused to quieten the build.

../drivers/input/misc/hp_sdc_rtc.c:268:12: warning: 'hp_sdc_rtc_proc_show' defined but not used [-Wunused-function]
  268 | static int hp_sdc_rtc_proc_show(struct seq_file *m, void *v)
      |            ^~~~~~~~~~~~~~~~~~~~

Fixes: c18bd9a1ff47 ("hp_sdc_rtc: Don't use create_proc_read_entry()")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/20230209010125.23690-1-rdunlap@infradead.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>