net: dwc-xlgmac: include dcbnl.h

net: dwc-xlgmac: include dcbnl.h

Without this header, we can run into a build error:

drivers/net/ethernet/synopsys/dwc-xlgmac-hw.c: In function 'xlgmac_config_queue_mapping':
drivers/net/ethernet/synopsys/dwc-xlgmac-hw.c:1548:36: error: 'IEEE_8021QAZ_MAX_TCS' undeclared (first use in this function)
  prio_queues = min_t(unsigned int, IEEE_8021QAZ_MAX_TCS,

Fixes: 65e0ace2c5cd ("net: dwc-xlgmac: Initial driver for DesignWare Enterprise Ethernet")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Jie Deng <jiedeng@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>