🔧 Sanity-check Mixing plus Disable Inactive Extruder

See #22166

Marlin/src/inc/SanityCheck.h

     #error "Please select either MIXING_EXTRUDER or SWITCHING_EXTRUDER, not both."
   #elif ENABLED(SINGLENOZZLE)
     #error "MIXING_EXTRUDER is incompatible with SINGLENOZZLE."
+  #elif ENABLED(DISABLE_INACTIVE_EXTRUDER)
+    #error "MIXING_EXTRUDER is incompatible with DISABLE_INACTIVE_EXTRUDER."
   #endif
 #endif