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/.gitmodules

@@ -0,0 +1,6 @@
+[submodule "contrib/DNS-LDNS"]
+	path = contrib/DNS-LDNS
+	url = https://github.com/erikoest/DNS-LDNS
+[submodule "test/tpkg"]
+	path = test/tpkg
+	url = https://github.com/NLnetLabs/tpkg.git