Bison parser cleanups: define yyerror() instead of MYSQLerror()/ORAerror()

To reduce the difference between sql_yacc.yy and sql_yacc_ora.yy,
using yyerror() in both files, instead of MYSQLerror() and ORAerror().
The pre-processor replaces yyerror() to MYSQLerror() and ORAerror()
anyway.

sql/sql_yacc.yy

@@ -136,7 +136,7 @@ int yylex(void *yylval, void *yythd);
   to abort from the parser.
 */
 
-void MYSQLerror(THD *thd, const char *s)
+static void yyerror(THD *thd, const char *s)
 {
   /*
     Restore the original LEX if it was replaced when parsing

sql/sql_yacc_ora.yy

@@ -136,7 +136,7 @@ int yylex(void *yylval, void *yythd);
   to abort from the parser.
 */
 
-void ORAerror(THD *thd, const char *s)
+static void yyerror(THD *thd, const char *s)
 {
   /*
     Restore the original LEX if it was replaced when parsing