errors: indent "symbol redeclaration" context

errors: indent "symbol redeclaration" context

This is the continuation of the work on the readability of errors
context.

For example, what used to be:
  input.y:5.10-24: %printer redeclaration for <field2>
  input.y:3.11-25: previous declaration

is now:
  input.y:5.10-24: %printer redeclaration for <field2>
  input.y:3.11-25:     previous declaration

* NEWS: Document this change.
* src/symtab.c (symbol_redeclaration, semantic_type_redeclaration,
user_token_number_redeclaration, default_tagged_destructor_set,
default_tagless_destructor_set, default_tagged_printer_set,
default_tagless_printer_set): Use complain_at_indent to
output with increased indentation level.
* tests/input.at: Apply this change.

Signed-off-by: Akim Demaille <akim@lrde.epita.fr>