Add Chitu 3D V6 2nd Filament Runout pin (#18922)

Marlin/src/pins/stm32f1/pins_CHITU3D_V6.h

 #define BEEPER_PIN                          PB0
 //#define LED_PIN                           PD3
 //#define POWER_LOSS_PIN                    PG2   // PG4 PW_DET
-#define FIL_RUNOUT_PIN                      PA15  // MT_DET
+
+#ifndef FIL_RUNOUT_PIN
+  #define FIL_RUNOUT_PIN                    PA15  // MT_DET
+#endif
+#ifndef FIL_RUNOUT2_PIN
+  #define FIL_RUNOUT2_PIN                   PF13
+#endif
 
 //
 // TronXY TFT Support