passt.h: Include netinet/if_ether.h before struct ctx declaration

This saves some hassle when including passt.h, as we need ETH_ALEN
there.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

passt.h

@@ -133,6 +133,8 @@ struct ip6_ctx {
 	struct in6_addr dns_fwd;
 };
 
+#include <netinet/if_ether.h>
+
 /**
  * struct ctx - Execution context
  * @mode:		Operation mode, qemu/UNIX domain socket or namespace/tap