mfd: mt6397: Add mutex include

Add missing mutex.h.
Signed-off-by: Josef Friedl <josef.friedl@speed.at>
Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

include/linux/mfd/mt6397/core.h

@@ -7,6 +7,8 @@
 #ifndef __MFD_MT6397_CORE_H__
 #define __MFD_MT6397_CORE_H__
 
+#include <linux/mutex.h>
+
 enum chip_id {
 	MT6323_CHIP_ID = 0x23,
 	MT6391_CHIP_ID = 0x91,