AP_Vehicle: expose LAND_FLARE_SEC in vehicle parameters for fixed wing

libraries/AP_Vehicle/AP_Vehicle.h

@@ -38,6 +38,7 @@ public:
         AP_Int16 pitch_limit_min_cd;        
         AP_Int8  autotune_level;
         AP_Int16 land_pitch_cd;
+        AP_Float land_flare_sec;
         AP_Int8  stall_prevention;
     };