ethtool: Fix includes build break

Based upon a patch by Oliver Hartkopp <oliver@hartkopp.net>.
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/ethtool.h

@@ -14,7 +14,6 @@
 #define _LINUX_ETHTOOL_H
 
 #include <linux/types.h>
-#include <linux/rculist.h>
 
 /* This should work for both 32 and 64 bit userland. */
 struct ethtool_cmd {
@@ -409,6 +408,8 @@ struct ethtool_flash {
 
 #ifdef __KERNEL__
 
+#include <linux/rculist.h>
+
 struct ethtool_rx_ntuple_flow_spec_container {
 	struct ethtool_rx_ntuple_flow_spec fs;
 	struct list_head list;