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/compat/malloc.c

@@ -0,0 +1,22 @@
+/* Just a replacement, if the original malloc is not
+   GNU-compliant. See autoconf documentation. */
+
+#if HAVE_CONFIG_H
+#include <ldns/config.h>
+#endif
+#undef malloc
+
+#include <sys/types.h>
+
+void *malloc (size_t n);
+
+/* Allocate an N-byte block of memory from the heap.
+   If N is zero, allocate a 1-byte block.  */
+
+void *
+rpl_malloc (size_t n)
+{
+  if (n == 0)
+    n = 1;
+  return malloc (n);
+}