Fix unused variable warnings (#14643)

Marlin/src/lcd/extensible_ui/ui_api.cpp

             && (linked_nozzles || active_extruder == 0)
           #endif
         ) zprobe_zoffset += mm;
+      #else
+        UNUSED(mm);
       #endif
 
       #if EXTRUDERS > 1
         }
       #else
         UNUSED(linked_nozzles);
+        UNUSED(mm);
       #endif
     }