make toString const

stepmania/src/RageInputDevice.cpp

@@ -305,7 +305,7 @@ DeviceButton StringToDeviceButton( InputDevice device, const CString& s )
 	}
 }
 
-CString DeviceInput::toString() 
+CString DeviceInput::toString() const
 {
 	if( device == DEVICE_NONE )
 		return "";

stepmania/src/RageInputDevice.h

@@ -359,7 +359,7 @@ public:
 		return ! operator==( other );
 	}
 	
-	CString toString();
+	CString toString() const;
 	bool fromString( const CString &s );
 
 	bool IsValid() const { return device != DEVICE_NONE; };