`IPhysicsObject::GetBuoyancyRatio` is not exposed as a virtual function

[RaphaelIT7]

In newer vphysics builds the GetBuoyancyRatio function was added which would probably be nice to have here too to be closer to the sdk :)
https://github.com/ValveSoftware/source-sdk-2013/blob/22288b919617be6c8ca3cefd7cca979cbb39a88c/src/public/vphysics_interface.h#L857