Move drivetrain_input_reader_ to a member variable for y2018

Someone missed the class.  Since there is only ever 1 reader at a
time, it was never caught.

Change-Id: I870a83c3b43941b453742be5112efcb7cc89b243
diff --git a/y2018/joystick_reader.cc b/y2018/joystick_reader.cc
index bc43b0c..41efc00 100644
--- a/y2018/joystick_reader.cc
+++ b/y2018/joystick_reader.cc
@@ -85,8 +85,6 @@
 const ButtonLocation kClawOpen(4, 4);
 const ButtonLocation kDriverClawOpen(2, 4);
 
-std::unique_ptr<DrivetrainInputReader> drivetrain_input_reader_;
-
 class Reader : public ::aos::input::JoystickInput {
  public:
   Reader() {
@@ -400,6 +398,8 @@
   ::aos::common::actions::ActionQueue action_queue_;
 
   ::std::unique_ptr<ProtoTXUdpSocket<VisionControl>> video_tx_;
+
+  ::std::unique_ptr<DrivetrainInputReader> drivetrain_input_reader_;
 };
 
 }  // namespace joysticks