Fix mixing with "unload all" compile

Marlin/src/inc/Conditionals_adv.h

@@ -56,6 +56,10 @@
   #undef SHOW_TEMP_ADC_VALUES
 #endif
 
+#if ENABLED(MIXING_EXTRUDER) && (ENABLED(RETRACT_SYNC_MIXING) || BOTH(FILAMENT_LOAD_UNLOAD_GCODES, FILAMENT_UNLOAD_ALL_EXTRUDERS))
+  #define HAS_MIXER_SYNC_CHANNEL 1
+#endif
+
 #if EITHER(DUAL_X_CARRIAGE, MULTI_NOZZLE_DUPLICATION)
   #define HAS_DUPLICATION_MODE 1
 #endif