SITL: fixed frame_property build error with older C++ compilers

libraries/SITL/SIM_Submarine.cpp

@@ -93,7 +93,7 @@ void Submarine::calculate_forces(const struct sitl_input &input, Vector3f &rot_a
 */
 float Submarine::calculate_buoyancy_acceleration()
 {
-    float below_water_level = position.z - frame_proprietary.height/2;
+    float below_water_level = position.z - frame_property.height/2;
 
     // Completely above water level
     if (below_water_level < 0) {
@@ -101,12 +101,12 @@ float Submarine::calculate_buoyancy_acceleration()
     }
 
     // Completely below water level
-    if (below_water_level > frame_proprietary.height/2) {
-        return frame_proprietary.bouyancy_acceleration;
+    if (below_water_level > frame_property.height/2) {
+        return frame_property.bouyancy_acceleration;
     }
 
     // bouyant force is proportional to fraction of height in water
-    return frame_proprietary.bouyancy_acceleration * below_water_level/frame_proprietary.height;
+    return frame_property.bouyancy_acceleration * below_water_level/frame_property.height;
 };
 
 /*

libraries/SITL/SIM_Submarine.h

@@ -45,9 +45,7 @@ public:
 protected:
     const float water_density = 1023.6; // (kg/m^3) At a temperature of 25 °C, salinity of 35 g/kg and 1 atm pressure
 
-    const class FrameConfig {
-    public:
-        FrameConfig() = default;
+    const struct {
         float length = 0.457; // x direction (meters)
         float width  = 0.338; // y direction (meters)
         float height = 0.254; // z direction (meters)
@@ -55,7 +53,7 @@ protected:
         float net_bouyancy = 2.0; // (N)
 
         float bouyancy_acceleration = GRAVITY_MSS + net_bouyancy/weight;
-    } frame_proprietary;
+    } frame_property;
 
     bool on_ground() const override;