Added z_fade_height to EEPROM-Settings

Firmware was forgetting the z_fade_height in Planner, set by M420 Z<f>,
after each reset or loading settings from EEPROM. Added the float
z_fade_height to EEPROM-Storage, now remembers the value.

Marlin/Marlin.h

@@ -307,6 +307,10 @@ float code_value_temp_diff();
   void reset_bed_level();
 #endif
 
+#if ENABLED(ENABLE_LEVELING_FADE_HEIGHT)
+  void set_z_fade_height(const float zfh);
+#endif
+
 #if ENABLED(Z_DUAL_ENDSTOPS)
   extern float z_endstop_adj;
 #endif