ALSA: Fix declaration of sound_class

Include sound/core.h in sound_core.c so that sound_class is declared
before it is defined, avoiding it looking like it should be static.
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/sound_core.c

@@ -10,6 +10,7 @@
 #include <linux/module.h>
 #include <linux/device.h>
 #include <linux/err.h>
+#include <sound/core.h>
 
 #ifdef CONFIG_SOUND_OSS_CORE
 static int __init init_oss_soundcore(void);