ASoC: fsl: Add explicit include of of.h

Hopefully fixing a build failure reported by Stephen Rothwell - though
quite why the other OF headers don't include this as well I'm not sure.
Signed-off-by: Mark Brown <broonie@linaro.org>

sound/soc/fsl/fsl_ssi.c

@@ -40,6 +40,7 @@
 #include <linux/delay.h>
 #include <linux/slab.h>
 #include <linux/spinlock.h>
+#include <linux/of.h>
 #include <linux/of_address.h>
 #include <linux/of_irq.h>
 #include <linux/of_platform.h>