MAINTAINERS: add myself as co-maintainer for r8169

Meanwhile I know the driver quite well and I refactored bigger parts
of it. As a result people contact me already with r8169 questions.
Therefore I'd volunteer to become co-maintainer of the driver also
officially.
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

MAINTAINERS

@@ -180,6 +180,7 @@ F:	drivers/net/hamradio/6pack.c
 
 8169 10/100/1000 GIGABIT ETHERNET DRIVER
 M:	Realtek linux nic maintainers <nic_swsd@realtek.com>
+M:	Heiner Kallweit <hkallweit1@gmail.com>
 L:	netdev@vger.kernel.org
 S:	Maintained
 F:	drivers/net/ethernet/realtek/r8169.c