Allow FIL_RUNOUT_PIN override on RAMPS-based boards

Marlin/src/pins/pins_RAMPS.h

@@ -257,7 +257,9 @@
 #endif
 
 // define digital pin 4 for the filament runout sensor. Use the RAMPS 1.4 digital input 4 on the servos connector
-#define FIL_RUNOUT_PIN      4
+#ifndef FIL_RUNOUT_PIN
+  #define FIL_RUNOUT_PIN    4
+#endif
 
 #ifndef PS_ON_PIN
   #define PS_ON_PIN        12