Add OV() macro for use in thermistor tables

Marlin/src/module/thermistor/thermistor_1.h

Marlin/src/module/thermistor/thermistor_10.h

Marlin/src/module/thermistor/thermistor_11.h

Marlin/src/module/thermistor/thermistor_12.h

Marlin/src/module/thermistor/thermistor_13.h

Marlin/src/module/thermistor/thermistor_2.h

Marlin/src/module/thermistor/thermistor_20.h

Marlin/src/module/thermistor/thermistor_3.h

Marlin/src/module/thermistor/thermistor_4.h

Marlin/src/module/thermistor/thermistor_5.h

Marlin/src/module/thermistor/thermistor_51.h

Marlin/src/module/thermistor/thermistor_52.h

Marlin/src/module/thermistor/thermistor_55.h

Marlin/src/module/thermistor/thermistor_6.h

Marlin/src/module/thermistor/thermistor_60.h

Marlin/src/module/thermistor/thermistor_66.h

Marlin/src/module/thermistor/thermistor_7.h

Marlin/src/module/thermistor/thermistor_70.h

Marlin/src/module/thermistor/thermistor_71.h

Marlin/src/module/thermistor/thermistor_8.h