Sanity check SWITCHING_EXTRUDER with SINGLENOZZLE

Marlin/src/inc/SanityCheck.h

 /**
  * Single Stepper Dual Extruder with switching servo
  */
-#if ENABLED(SWITCHING_EXTRUDER) && NUM_SERVOS < 1
-  #error "SWITCHING_EXTRUDER requires 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
 
 /**