sensor_max6675: mark as untested for STM32F4

lib/sensor_max6675.h

@@ -6,6 +6,7 @@
  *  @note peripherals used: SPI @ref sensor_max6675_spi
  */
 #pragma once
+#error not converted for STM32F4
 
 /** setup communication to MAX6675 sensor */
 void sensor_max6675_setup(void);