Fix GTM32_PRO_VB Serial and HAL via watchdog (#12753)

Move `watchdog_reset` from `.h` to `.cpp` to eliminate the need to include `MarlinConfig.h` in a HAL-related file.

Marlin/src/HAL/HAL_STM32F1/watchdog_STM32F1.cpp

@@ -33,6 +33,13 @@
 #include <libmaple/iwdg.h>
 #include "watchdog_STM32F1.h"
 
+void watchdog_reset() {
+  #if PIN_EXISTS(LED)
+    TOGGLE(LED_PIN);  // heartbeat indicator
+  #endif
+  iwdg_feed();
+}
+
 void watchdogSetup(void) {
   // do whatever. don't remove this function.
 }