intl/plural.y: Avoid conflicting declarations of yyerror and yylex

bison-3.8 includes these lines in the generated intl/plural.c:

  #if !defined __gettexterror && !defined YYERROR_IS_DECLARED
  void __gettexterror (struct parse_args *arg, const char *msg);
  #endif
  #if !defined __gettextlex && !defined YYLEX_IS_DECLARED
  int __gettextlex (YYSTYPE *yylvalp, struct parse_args *arg);
  #endif

Those default prototypes provided by bison conflict with the
declarations later on in plural.y.  This patch solves the issue.

Reviewed-by: Arjun Shankar <arjun@redhat.com>