fix: populate ldns submodule and add autotools to LDNS build stage

- Re-cloned zonemaster-ldns with --recurse-submodules so the bundled
  ldns C library source (including Changelog and configure.ac) is present
- Added autoconf, automake, libtool to Dockerfile.backend ldns-build stage
  so libtoolize + autoreconf can generate ldns/configure during make

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

zonemaster-ldns/ldns/examples/ldns-version.c

@@ -0,0 +1,16 @@
+/*
+ * ldns-version shows ldns's version 
+ *
+ * (c) NLnet Labs, 2005 - 2008
+ * See the file LICENSE for the license
+ */
+
+#include "config.h"
+#include <ldns/ldns.h>
+
+int
+main(void)
+{
+	printf("%s\n", ldns_version());
+        return 0;
+}