LibC: Add the netinet/if_ether.h header and define ETH_ALEN

This header defines various constants related to ethernet packets,
with ETH_ALEN being the length of an ethernet address (which is needed
by some ports).

Userland/Libraries/LibC/netinet/if_ether.h

@@ -0,0 +1,9 @@
+/*
+ * Copyright (c) 2021, Idan Horowitz <idan.horowitz@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#define ETH_ALEN 6 // Length of an ethernet address (MAC) in bytes