net: Fix build error when DEBUG_NET is on

"qemu-common.h" should be included to provide the forward declaration
of qemu_hexdump() when DEBUG_NET is on.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>