Sanity check SWITCHING_EXTRUDER with SINGLENOZZLE

Marlin/src/inc/SanityCheck.h

@@ -582,9 +582,13 @@ static_assert(X_MAX_LENGTH >= X_BED_SIZE && Y_MAX_LENGTH >= Y_BED_SIZE,
 /**
  * Single Stepper Dual Extruder with switching servo
  */
-#if ENABLED(SWITCHING_EXTRUDER) && NUM_SERVOS < 1
+#if ENABLED(SWITCHING_EXTRUDER)
+  #if ENABLED(SINGLENOZZLE)
+    #error "SWITCHING_EXTRUDER and SINGLENOZZLE are incompatible."
+  #elif NUM_SERVOS < 1
     #error "SWITCHING_EXTRUDER requires NUM_SERVOS >= 1."
   #endif
+#endif
 
 /**
  * Mixing Extruder requirements