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/lua/rdf.lua

@@ -0,0 +1,7 @@
+-- test rdf stuff
+
+dofile("rns-lib.lua")
+
+rdf = l_rdf_new_frm_str(LDNS_RDF_TYPE_DNAME, "miek.nl")
+l_rdf_print(rdf)
+print()