Fix Z_PROBE_PING not declared, allows code to compile if Z_PROBE_ENDSTOP

is not used.

Marlin/pins.h

@@ -187,7 +187,7 @@
   #define Z_MIN_PIN          -1
 #endif
 
-#ifdef DISABLE_Z_PROBE_ENDSTOP
+#if defined (DISABLE_Z_PROBE_ENDSTOP) || ! defined (Z_PROBE_ENDSTOP) // Allow code to compile regardless of Z_PROBE_ENDSTOP setting.
   #define Z_PROBE_PIN        -1
 #endif