Fix SKEW_CORRECTION without leveling

Marlin/planner.h

@@ -170,6 +170,8 @@ class Planner {
       #if ENABLED(ENABLE_LEVELING_FADE_HEIGHT)
         static float z_fade_height, inverse_z_fade_height;
       #endif
+    #else
+      static constexpr bool leveling_active = false;
     #endif
 
     #if ENABLED(LIN_ADVANCE)