ALSA: ac97 - Include ac97_codec.h for ac97_bus_type declaration

This fixes a sparse warning caused by the lack of a connection with the
prototype for ac97_bus_type.
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/ac97_bus.c

@@ -15,6 +15,7 @@
 #include <linux/init.h>
 #include <linux/device.h>
 #include <linux/string.h>
+#include <sound/ac97_codec.h>
 
 /*
  * Let drivers decide whether they want to support given codec from their