r8169: add missing Kconfig dependency

Now that we switched the r8169 driver to use phylib, there's a
dependency on the Realtek PHY drivers. This dependency was missing
in Kconfig.

Reported-by: Jouni Mettälä <jtmettala@gmail.com>
Fixes: f1e911d5 ("r8169: add basic phylib support")
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>