net: au1000_eth: add missing capability.h

fixes the following build failure:
  CC      drivers/net/au1000_eth.o
/drivers/net/au1000_eth.c: In function 'au1000_set_settings':
/drivers/net/au1000_eth.c:623: error: implicit declaration of function 'capable'
/drivers/net/au1000_eth.c:623: error: 'CAP_NET_ADMIN' undeclared (first use in this function)
/drivers/net/au1000_eth.c:623: error: (Each undeclared identifier is reported only once
/drivers/net/au1000_eth.c:623: error: for each function it appears in.

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>