[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>

include/sound/core.h

@@ -339,6 +339,7 @@ unsigned int snd_dma_pointer(unsigned long dma, unsigned int size);
 #endif
 
 /* misc.c */
+struct resource;
 void release_and_free_resource(struct resource *res);
 
 #ifdef CONFIG_SND_VERBOSE_PRINTK