gc: line number bug fix

R=ken2
CC=golang-dev
https://golang.org/cl/1243044

src/cmd/gc/walk.c

@@ -221,7 +221,7 @@ walkdef(Node *n)
 	if(n->op == ONONAME) {
 		if(!n->diag) {
 			n->diag = 1;
-			yyerror("undefined: %S", n->sym);
+			yyerrorl(n->lineno, "undefined: %S", n->sym);
 		}
 		return;
 	}