LibC: Add definitions/stubs for IPv6 functionality

This lets us build some of the ports without patching out any of their
IPv6 stuff.

Userland/Libraries/LibC/net.cpp

@@ -0,0 +1,23 @@
+/*
+ * Copyright (c) 2021, Gunnar Beutner <gbeutner@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibC/errno.h>
+#include <LibC/net/if.h>
+#include <LibC/netinet/in.h>
+
+in6_addr in6addr_any = IN6ADDR_ANY_INIT;
+
+unsigned int if_nametoindex([[maybe_unused]] const char* ifname)
+{
+    errno = ENODEV;
+    return -1;
+}
+
+char* if_indextoname([[maybe_unused]] unsigned int ifindex, [[maybe_unused]] char* ifname)
+{
+    errno = ENXIO;
+    return nullptr;
+}