ASoC: Fix S/PDIF build

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
diff --git a/sound/soc/codecs/spdif_transciever.c b/sound/soc/codecs/spdif_transciever.c
index f0945ab..9119836 100644
--- a/sound/soc/codecs/spdif_transciever.c
+++ b/sound/soc/codecs/spdif_transciever.c
@@ -16,6 +16,7 @@
 
 #include <linux/module.h>
 #include <linux/moduleparam.h>
+#include <linux/slab.h>
 #include <sound/soc.h>
 #include <sound/pcm.h>
 #include <sound/initval.h>