rfkill: include err.h

Since we use ERR_PTR and similar macros, we need to include
linux/err.h.
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/rfkill.h

@@ -105,6 +105,7 @@ enum rfkill_user_states {
 #include <linux/mutex.h>
 #include <linux/device.h>
 #include <linux/leds.h>
+#include <linux/err.h>
 
 /* this is opaque */
 struct rfkill;