net/dsa/realtek: add MODULE_LICENSE()

Add MODULE_LICENSE() to net/dsa/realtek.o to fix build warning message.

WARNING: modpost: missing MODULE_LICENSE() in drivers/net/dsa/realtek.o

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>