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/contrib/python/examples/python3/ldns-buf.py

@@ -0,0 +1,8 @@
+#!/usr/bin/python
+
+import ldns
+
+buf = ldns.ldns_buffer(1024)
+buf.printf("Test buffer")
+print(buf)
+