Don't consider a vcc expr to be constant after vcc_expr_tostring.

Avoids vcc errors from constructs like "std.log("Test: " + 1);"

Fixes: #1134

bin/varnishtest/tests/r01134.vtc

+varnishtest "vcc const/non-const tostring conversion - bug 1134"
+
+server s1 {
+	rxreq
+	txresp
+} -start
+
+varnish v1 -vcl+backend {
+	sub vcl_recv {
+		set req.http.x-test = "Test " + 1;
+	}
+} -start
+
+client c1 {
+	txreq
+	rxresp
+	expect resp.status == 200
+} -run
+

lib/libvcl/vcc_expr.c

@@ -434,8 +434,10 @@ vcc_expr_tostring(struct expr **e, enum var_type fmt)
 	case TIME:	p = "VRT_time_string(sp, \v1)"; break;
 	default:	break;
 	}
-	if (p != NULL)
+	if (p != NULL) {
 		*e = vcc_expr_edit(fmt, p, *e, NULL);
+		(*e)->constant = 0;
+	}
 }
 
 /*--------------------------------------------------------------------