Fix include order to make SERIAL_PORT_2 work (#13241)

Marlin/src/HAL/HAL_STM32/HAL.h

 #include "fastio_STM32.h"
 #include "watchdog_STM32.h"
 
-#include "HAL_timers_STM32.h"
-
 // --------------------------------------------------------------------------
 // Defines
 // --------------------------------------------------------------------------
   #define NUM_SERIAL 1
 #endif
 
+#include "HAL_timers_STM32.h"
+
 /**
  * TODO: review this to return 1 for pins that are not analog input
  */