Also fix temperature externs

Marlin/temperature.h

 
 // low level conversion routines
 // do not use these routines and variables outside of temperature.cpp
-extern int target_temperature[EXTRUDERS];  
-extern float current_temperature[EXTRUDERS];
+extern int target_temperature[4];  
+extern float current_temperature[4];
 #ifdef SHOW_TEMP_ADC_VALUES
-  extern int current_temperature_raw[EXTRUDERS];
+  extern int current_temperature_raw[4];
   extern int current_temperature_bed_raw;
 #endif
 extern int target_temperature_bed;