Fix SINGLENOZZLE fan speed bug (#19152)

Marlin/src/module/temperature.cpp

         if (target < EXTRUDERS) singlenozzle_fan_speed[target] = speed;
         return;
       }
-      target = 0; // Always use fan index 0 with SINGLENOZZLE
     #endif
 
+    TERN_(SINGLENOZZLE, target = 0); // Always use fan index 0 with SINGLENOZZLE
+
     if (target >= FAN_COUNT) return;
 
     fan_speed[target] = speed;