Added POV to joysticks.

Change-Id: Iad0ea8ed1526a6173502f3f0a9bc8918f0569adc
diff --git a/aos/common/messages/robot_state.q b/aos/common/messages/robot_state.q
index 74fe5d0..3ecb653 100644
--- a/aos/common/messages/robot_state.q
+++ b/aos/common/messages/robot_state.q
@@ -6,6 +6,9 @@
 
   // The 4 joystick axes.
   double[4] axis;
+
+  // The POV axis.
+  int32_t pov;
 };
 
 message JoystickState {