AP_HAL: use singleton getter for Baro in SIMState

this instance variable was always nullptr due to constructor ordering

libraries/AP_HAL/SIMState.cpp

@@ -43,9 +43,6 @@ void SIMState::_sitl_setup(const char *home_str)
     _home_str = home_str;
 
     printf("Starting SITL input\n");
-
-    // find the barometer object if it exists
-    _barometer = AP_Baro::get_singleton();
 }
 
 
@@ -235,6 +232,9 @@ void SIMState::_simulator_servos(struct sitl_input &input)
     uint32_t now = AP_HAL::micros();
     // last_update_usec = now;
 
+    // find the barometer object if it exists
+    const auto *_barometer = AP_Baro::get_singleton();
+
     float altitude = _barometer?_barometer->get_altitude():0;
     float wind_speed = 0;
     float wind_direction = 0;

libraries/AP_HAL/SIMState.h

@@ -87,8 +87,6 @@ private:
     pid_t _parent_pid;
     uint32_t _update_count;
 
-    class AP_Baro *_barometer;
-
 #if CONFIG_HAL_BOARD == HAL_BOARD_SITL
     SocketAPM _sitl_rc_in{true};
 #endif