[airborne] don't call uart_event() in simulators

uart_event was added as a weak function with #951, but it is not provided in simulators.
The compiler (correctly) fills in a zero and the simulator segfaults.

For now simply fix it by not calling uart_event in simulators.

sw/airborne/firmwares/fixedwing/main_fbw.c

@@ -189,7 +189,9 @@ void event_task_fbw( void) {
   i2c_event();
 #endif
 
+#ifndef SITL
   uart_event();
+#endif
 
 #ifdef INTER_MCU
 #if defined MCU_SPI_LINK | defined MCU_UART_LINK

sw/airborne/firmwares/rotorcraft/main.c

@@ -275,7 +275,9 @@ STATIC_INLINE void main_event( void ) {
 
   i2c_event();
 
+#ifndef SITL
   uart_event();
+#endif
 
 #if USE_UDP
   udp_event();