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/codingstyle.tpkg/codingstyle.dsc

@@ -0,0 +1,16 @@
+BaseName: codingstyle
+Version: 1.0
+Description: Check LDNS CodingStyle
+CreationDate: Tue Mar 14 12:58:28 CET 2006
+Maintainer: Miek Gieben
+Category: 
+Component:
+CmdDepends: 
+Depends: 
+Help: codingstyle.help
+Pre: 
+Post: 
+Test: codingstyle.test
+AuxFiles: 
+Passed:
+Failure: