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 <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Takashi Iwai <[e-mail]>

include/sound/gus.h

···
 int snd_gus_dram_read(struct snd_gus_card *gus, char __user *ptr,
 		      unsigned int addr, unsigned int size, int rom);
 
+/* gus_timer.c */
+void snd_gf1_timers_init(struct snd_gus_card *gus);
+void snd_gf1_timers_done(struct snd_gus_card *gus);
+
 #endif /* __SOUND_GUS_H */

sound/isa/gus/gus_reset.c

···
 #include <sound/core.h>
 #include <sound/gus.h>
 
-extern void snd_gf1_timers_init(struct snd_gus_card * gus);
-extern void snd_gf1_timers_done(struct snd_gus_card * gus);
 extern int snd_gf1_synth_init(struct snd_gus_card * gus);
 extern void snd_gf1_synth_done(struct snd_gus_card * gus);