atkobject: fix get_property for accessible-role
atkobject: fix get_property for accessible-role

Previous commit fixed set_property for accessible-role (as it is a
enum instead of an int). As Rico Tzschichholz pointed on bug 794513,
get_property was not updated accordingly.

GNOME/at-spi2-core,GNOME/at-spi2-core,GNOME/at-spi2-core