Fix timers.h include

Marlin/src/HAL/HAL_STM32/SoftwareSerial.cpp

 // Includes
 //
 #include "SoftwareSerial.h"
-#include <timer.h>
+#include "timers.h"
 
 #define OVERSAMPLE 3 // in RX, Timer will generate interruption OVERSAMPLE time during a bit. Thus OVERSAMPLE ticks in a bit. (interrupt not synchonized with edge).