Add definition for TIMER_SERIAL, so it does not collide with TIMER_SERVO

Author: ghent360

Diff for: variants/PRNTR_V2/variant.h

@@ -150,6 +150,7 @@ extern "C" {
 // Use TIM6/TIM7 when possible as servo and tone don't need GPIO output pin
 #define TIMER_TONE              TIM6
 #define TIMER_SERVO             TIM7
+#define TIMER_SERIAL            TIM12
 
 // UART Definitions
 #define SERIAL_UART_INSTANCE    1