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>
diff --git a/src/yacc_config.y b/src/yacc_config.y
index ce14729..d198200 100644
--- a/src/yacc_config.y
+++ b/src/yacc_config.y
@@ -27,7 +27,7 @@
 extern char *current_file;
 extern int current_lineno;
 
-void yyerror(char *msg, ...);
+void yyerror(char const *msg, ...);
 int yylex(void);
 
 %}
@@ -113,7 +113,7 @@
 	;
 
 %%
-void yyerror(char *msg, ...)
+void yyerror(char const *msg, ...)
 {
      va_list ap;
      char str[256];