Fix compile without ENABLE_LEVELING_FADE_HEIGHT

Marlin/planner.cpp

@@ -552,6 +552,8 @@ void Planner::check_axes_activity() {
       }
       else
         z_fade_factor = 1.0;
+    #else
+        constexpr float z_fade_factor = 1.0;
     #endif
 
     #if ENABLED(MESH_BED_LEVELING)