[wpilibc] MotorSafety::GetDescription(): Return std::string (#3390)

This is only called in an error condition, so it's not necessary to over
optimize it.

wpilibc/src/main/native/cpp/drive/DifferentialDrive.cpp

@@ -189,8 +189,8 @@ void DifferentialDrive::StopMotor() {
   Feed();
 }
 
-void DifferentialDrive::GetDescription(wpi::raw_ostream& desc) const {
-  desc << "DifferentialDrive";
+std::string DifferentialDrive::GetDescription() const {
+  return "DifferentialDrive";
 }
 
 void DifferentialDrive::InitSendable(SendableBuilder& builder) {