net: fix build failure in xilinx

Commit 71c6c837 (drivers/net: fix tasklet misuse issue) introduced a
build failure in the xilinx driver. axienet_dma_err_handler isn't
declared before its use in axienet_open.

This patch provides the prototype before axienet_open.

Cc: Xiaotian Feng <dannyfeng@tencent.com>
Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/xilinx/xilinx_axienet_main.c

@@ -894,6 +894,8 @@ out:
 	return IRQ_HANDLED;
 }
 
+static void axienet_dma_err_handler(unsigned long data);
+
 /**
  * axienet_open - Driver open routine.
  * @ndev:	Pointer to net_device structure