i2c: mux: add sysfs header

We are using sysfs functions directly, so we should include the header.
Signed-off-by: NWolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: NPeter Rosin <peda@axentia.se>

drivers/i2c/i2c-mux.c

@@ -26,6 +26,7 @@
 #include <linux/module.h>
 #include <linux/of.h>
 #include <linux/slab.h>
+#include <linux/sysfs.h>
 
 /* multiplexer per channel data */
 struct i2c_mux_priv {