[media] s5p-tv: Include missing irqreturn.h header

Without this patch we get the following compilation errors:
drivers/media/platform/s5p-tv/mixer.h:345:13: error: Expected ; at end of declaration
drivers/media/platform/s5p-tv/mixer.h:345:13: error: got mxr_irq_handler
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/platform/s5p-tv/mixer.h

@@ -19,6 +19,7 @@
 #endif
 
 #include <linux/fb.h>
+#include <linux/irqreturn.h>
 #include <linux/kernel.h>
 #include <linux/spinlock.h>
 #include <linux/wait.h>