ARM: highbank: fix warning for calxedaxgmac_initialize

Add include of netdev.h to pick-up declaration of calxedaxgmac_initialize.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
diff --git a/board/highbank/highbank.c b/board/highbank/highbank.c
index b0aa182..5f34fec 100644
--- a/board/highbank/highbank.c
+++ b/board/highbank/highbank.c
@@ -17,6 +17,7 @@
 
 #include <common.h>
 #include <ahci.h>
+#include <netdev.h>
 #include <scsi.h>
 
 #include <asm/sizes.h>