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/test/19-keygen.tpkg/19-keygen.dsc

@@ -0,0 +1,15 @@
+BaseName: 19-keygen
+Version: 1.0
+Description: Check the output of the key generator
+CreationDate: Mon Dec 10 13:52:06 CET 2007
+Maintainer: Jelte Jansen
+Category: 
+Component:
+Depends: 
+Help: 19-keygen.help
+Pre: 
+Post:
+Test: 19-keygen.test
+AuxFiles: 
+Passed:
+Failure: