msvc: added vd2 flag to a compilator

vd2 flag allows to use dynamic_cast in an object being constructed.
This flag is used in case where the only virtual function in virtual base
class is a destructor (so it is in ObjectBase class).

        * MSVC_2010/gstreamermm.props: added necessary flag in a property
          sheet file, so user who uses this property sheet in his
          project, don't have to remember about adding this flag.