Add xdg-su program to gparted.desktop file (#699626)

Add xdg-su to list of graphical switch-to-root programs to be
considered for gparted.desktop file.

The openSUSE GNU/Linux distribution includes the program xdg-su by
default for graphically prompting for root privilege.

This enhancement enables a user to compile and install gparted from
source code on openSUSE and have the gparted menu entry graphically
prompt for root privilege.

Bug #699626 - Enable gparted.desktop to prompt for root on default
              openSUSE installation