SITL: Create gps_hdt_enabled varible for enable/disable HDT GPS sentence

libraries/SITL/SITL.cpp

@@ -168,6 +168,8 @@ const AP_Param::GroupInfo SITL::var_info2[] = {
     AP_GROUPINFO("GND_BEHAV",   41, SITL,  gnd_behav, -1),
     AP_GROUPINFO("BARO_COUNT",  42, SITL,  baro_count,  1),
 
+    AP_GROUPINFO("GPS_HDG",     43, SITL,  gps_hdg_enabled, 0),
+
     AP_GROUPEND
 
 };

libraries/SITL/SITL.h

@@ -175,6 +175,7 @@ public:
     AP_Int8  telem_baudlimit_enable; // enable baudrate limiting on links
     AP_Float flow_noise; // optical flow measurement noise (rad/sec)
     AP_Int8  baro_count; // number of simulated baros to create
+    AP_Int8 gps_hdg_enabled; // enable the output of a NMEA heading HDT sentence
 
     // wind control
     enum WindType {