ui/ExperimentalButton: Initialize `engageable` to false (#28846)

selfdrive/ui/qt/onroad.cc

@@ -198,7 +198,7 @@ void OnroadAlerts::paintEvent(QPaintEvent *event) {
 }
 
 // ExperimentalButton
-ExperimentalButton::ExperimentalButton(QWidget *parent) : experimental_mode(false), QPushButton(parent) {
+ExperimentalButton::ExperimentalButton(QWidget *parent) : experimental_mode(false), engageable(false), QPushButton(parent) {
   setFixedSize(btn_size, btn_size);
 
   params = Params();