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/zone.txt

@@ -0,0 +1,15 @@
+$ORIGIN example.
+$TTL 600
+
+example.        IN SOA  example. admin.example. (
+                                2008022501 ; serial
+                                28800      ; refresh (8 hours)
+                                7200       ; retry (2 hours)
+                                604800     ; expire (1 week)
+                                18000      ; minimum (5 hours)
+                                )
+
+@       IN      MX      10 mail.example.
+@       IN      NS      ns1
+@       IN      NS      ns2
+@       IN      A       192.168.1.1