[wpilib] RobotController: Add isSystemTimeValid() (#5696)

This returns true when the system date/time (wall clock) is valid.

wpilibc/src/main/native/include/frc/RobotController.h

@@ -115,6 +115,13 @@ class RobotController {
    */
   static bool GetRSLState();
 
+  /**
+   * Gets if the system time is valid.
+   *
+   * @return True if the system time is valid, false otherwise
+   */
+  static bool IsSystemTimeValid();
+
   /**
    * Get the input voltage to the robot controller.
    *