errors: indent "user token number redeclaration" context

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

* src/symtab.c (user_token_number_redeclaration): 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>

src/symtab.c

@@ -534,6 +534,7 @@ user_token_number_redeclaration (int num, symbol *first, symbol *second)
   complain_at_indent (second->location, &i,
                       _("user token number %d redeclaration for %s"),
                       num, second->tag);
+  i += SUB_INDENT;
   complain_at_indent (first->location, &i,
                       _("previous declaration for %s"),
                       first->tag);