ALSA: isa/gus: remove -Wmissing-prototypes warnings

ALSA: isa/gus: remove -Wmissing-prototypes warnings

Fix W=1 warnings by adding prototypes to header file

sound/isa/gus/gus_timer.c:141:6: warning: no previous prototype for
‘snd_gf1_timers_init’ [-Wmissing-prototypes]
  141 | void snd_gf1_timers_init(struct snd_gus_card * gus)
      |      ^~~~~~~~~~~~~~~~~~~
sound/isa/gus/gus_timer.c:177:6: warning: no previous prototype for
‘snd_gf1_timers_done’ [-Wmissing-prototypes]
  177 | void snd_gf1_timers_done(struct snd_gus_card * gus)
      |      ^~~~~~~~~~~~~~~~~~~

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200702193604.169059-2-pierre-louis.bossart@linux.intel.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>