Fix extrude_min_temp compiler warning

Marlin/temperature.cpp

 
 #if ENABLED(PREVENT_COLD_EXTRUSION)
   bool Temperature::allow_cold_extrude = false;
-  uint16_t Temperature::extrude_min_temp = EXTRUDE_MINTEMP;
+  int16_t Temperature::extrude_min_temp = EXTRUDE_MINTEMP;
 #endif
 
 // private:

Marlin/temperature.h

 
     #if ENABLED(PREVENT_COLD_EXTRUSION)
       static bool allow_cold_extrude;
-      static uint16_t extrude_min_temp;
+      static int16_t extrude_min_temp;
       static bool tooColdToExtrude(uint8_t e) {
         #if HOTENDS == 1
           UNUSED(e);