const arguments to test_dual_z_endstops

Marlin/endstops.cpp

 #if ENABLED(Z_DUAL_ENDSTOPS)
 
   // Pass the result of the endstop test
-  void Endstops::test_dual_z_endstops(EndstopEnum es1, EndstopEnum es2) {
+  void Endstops::test_dual_z_endstops(const EndstopEnum es1, const EndstopEnum es2) {
     byte z_test = TEST_ENDSTOP(es1) | (TEST_ENDSTOP(es2) << 1); // bit 0 for Z, bit 1 for Z2
     if (z_test && stepper.current_block->steps[Z_AXIS] > 0) {
       SBI(endstop_hit_bits, Z_MIN);

Marlin/endstops.h

   private:
 
     #if ENABLED(Z_DUAL_ENDSTOPS)
-      static void test_dual_z_endstops(EndstopEnum es1, EndstopEnum es2);
+      static void test_dual_z_endstops(const EndstopEnum es1, const EndstopEnum es2);
     #endif
 };