Fix yyerror warning

y.tab.c: In function 'yyparse':
y.tab.c:1633: warning: passing argument 1 of 'yyerror' discards qualifiers from pointer target type
src/yacc_config.y:30: note: expected 'char *' but argument is of type 'const char *'

Add const in yyerror declaration the way it is in bison doc.
Should be fine as msg is not modified.

Signed-off-by: Gilles Espinasse <g.esp@free.fr>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>