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.

THANKS

@@ -46,8 +46,8 @@ Csaba Raduly              csaba_22@yahoo.co.uk
 Dagobert Michelsen        dam@baltic-online.de
 Daniel Frużyński          daniel@poradnik-webmastera.com
 Daniel Galloway           dg1751@att.com
-Daniela Becker            daniela@lrde.epita.fr
 Daniel Hagerty            hag@gnu.org
+Daniela Becker            daniela@lrde.epita.fr
 David Barto               david.barto@sparqlcity.com
 David J. MacKenzie        djm@gnu.org
 David Kastrup             dak@gnu.org
@@ -143,6 +143,7 @@ Nick Bowler               nbowler@elliptictech.com
 Nicolas Bedon             nicolas.bedon@univ-rouen.fr
 Nicolas Burrus            nicolas.burrus@epita.fr
 Nicolas Tisserand         nicolas.tisserand@epita.fr
+Nikita Popov              nikic@php.net
 Nikki Valen               nicolettavalencia.nv@gmail.com
 Noah Friedman             friedman@gnu.org
 Odd Arild Olsen           oao@fibula.no
@@ -170,7 +171,6 @@ Quoc Peyrot               chojin@lrde.epita.fr
 R Blake                   blakers@mac.com
 Raja R Harinath           harinath@cs.umn.edu
 Ralf Wildenhues           Ralf.Wildenhues@gmx.de
-Ryan                      dev@splintermail.com
 Rich Wilson               richaw@gmail.com
 Richard Stallman          rms@gnu.org
 Rici Lake                 ricilake@gmail.com
@@ -179,6 +179,7 @@ Rob Vermaas               rob.vermaas@gmail.com
 Robert Anisko             anisko_r@epita.fr
 Robert Yang               liezhi.yang@windriver.com
 Roland Levillain          roland@lrde.epita.fr
+Ryan                      dev@splintermail.com
 Satya Kiran Popuri        satyakiran@gmail.com
 Sebastian Setzer          sebastian.setzer.ext@siemens.com
 Sebastien Fricker         sebastien.fricker@gmail.com