parsers: issue tname with i18n markup

Some users would like to avoid having to "parse" the *.y file to find
the strings to translate.  Let's issue the translatable tokens with N_
to allow "parsing" the generated parsers instead.

See
https://lists.gnu.org/archive/html/bison-patches/2019-01/msg00015.html

* src/output.c (prepare_symbol_names): Issue symbol_names with N_()
markup.

tests/calc.at

@@ -422,6 +422,7 @@ void location_print (FILE *o, Span s);
   ]AT_YYLEX_DECLARE_EXTERN[
 
   ]AT_TOKEN_TRANSLATE_IF([[
+#define N_
     static
     const char *
     _ (const char *cp)