hdb: Remove default HDB backend footgun

Do not allow a change in build configuration time default HDB backend
selection cause existing default HDBs to not be possible to open.

Otherwise such a change will cause a KDC configured to use the default
HDB (i.e., without setting it in the "database" stanza in the "[kdc]"
section of krb5.conf) to not start.

lib/hdb/hdb-ldap.c

@@ -2092,6 +2092,7 @@ fini(void *ctx)
 
 struct hdb_method hdb_ldap_interface = {
     HDB_INTERFACE_VERSION,
+    0 /*is_file_based*/, 0 /*can_taste*/,
     init,
     fini,
     "ldap",