net: Use a forward declaration for cmd_tbl in net-common.h

We don't need to include command.h just for this declaration. It
eventually pulls in linux/byteorder which causes a conflict with exfat
which has its own byteorder functions.

Use a forward declaration instead.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/net-common.h

@@ -4,7 +4,6 @@
 #define __NET_COMMON_H__
 
 #include <asm/cache.h>
-#include <command.h>
 #include <env.h>
 #include <hexdump.h>
 #include <linux/if_ether.h>
@@ -13,6 +12,8 @@
 #include <rand.h>
 #include <time.h>
 
+struct cmd_tbl;
+
 #define DEBUG_NET_PKT_TRACE 0	/* Trace all packet data */
 
 /*