yacc.c, lalr1.cc: fix warnings about yynerrs

Most often yynerrs is not used.  Clang 15 now complains about such
variables.  Bison itself does not compile:

```
src/parse-gram.c:1797:9: error: variable 'gram_nerrs' set but not used [-Werror,-Wunused-but-set-variable]
    int yynerrs = 0;
        ^
src/parse-gram.c:79:25: note: expanded from macro 'yynerrs'
                        ^
1 error generated.
```

Reported by Nikita Popov.
Fixes https://github.com/akimd/bison/issues/89.

* data/skeletons/yacc.c (yynerrs): Flag with YY_ATTRIBUTE_UNUSED.
* data/skeletons/lalr1.cc (yynerrs_): Likewise.
3 files changed