Make G26 work with all mesh leveling.

Example Configuration.h files are not updated yet.   You need to cross
your settings over to the default Configuration.h file in the \Marlin
directory.   (UBL_G26_MESH_VALIDATION enablement has moved to a new
location in the file.)

Marlin/planner.cpp

@@ -58,6 +58,7 @@
  *
  */
 
+#include "MarlinConfig.h"
 #include "planner.h"
 #include "stepper.h"
 #include "temperature.h"