Add FIL_RUNOUT_PIN for BIGTREE_SKR_V1.3

See #13267

Marlin/src/pins/pins_BIGTREE_SKR_V1.3.h

 #endif
 
 //
+// Filament Runout Sensor
+//
+#ifndef FIL_RUNOUT_PIN
+  #define FIL_RUNOUT_PIN   P1_28
+#endif
+
+//
 // Steppers
 //
 #define X_STEP_PIN         P2_02