mfd: Add missing mutex.h inclusion to WM8994 core.h

struct wm8994 includes a mutex so we need to include mutex.h before we
declare it. All current users rely on this being done implicitly.
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>

include/linux/mfd/wm8994/core.h

@@ -15,6 +15,7 @@
 #ifndef __MFD_WM8994_CORE_H__
 #define __MFD_WM8994_CORE_H__
 
+#include <linux/mutex.h>
 #include <linux/interrupt.h>
 
 enum wm8994_type {