[ALSA] Add the missing forward declration

Modules: ALSA Core

Added the missing forward declaration before function prototypes.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/include/sound/core.h b/include/sound/core.h
index ed56a35..642ddfb 100644
--- a/include/sound/core.h
+++ b/include/sound/core.h
@@ -339,6 +339,7 @@
 #endif
 
 /* misc.c */
+struct resource;
 void release_and_free_resource(struct resource *res);
 
 #ifdef CONFIG_SND_VERBOSE_PRINTK