staging: mt7621-eth: Fix sparse warning in ethtool.c

This fixes the following sparse warning:

drivers/staging/mt7621-eth/ethtool.c:213:6: warning: symbol
'mtk_set_ethtool_ops' was not declared. Should it be static?
Signed-off-by: Chris Coffey <cmc@babblebit.net>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/mt7621-eth/ethtool.c

@@ -13,6 +13,7 @@
  */
 
 #include "mtk_eth_soc.h"
+#include "ethtool.h"
 
 static const char mtk_gdma_str[][ETH_GSTRING_LEN] = {
 #define _FE(x...)	# x,