xdg-utils is needed when using the graphical wireshark (enabled with USE=gtk), to find the html viewer. When building only the command line tool (tshark), the depend is absolutelly not needed, and add on headless servers TONS of X related dependancies. So xdg-utils should be inside gtk condition. Follow an ebuild patch
Created attachment 255995 [details, diff] move xdg dependancy
Thank you for report. Fixed.
*** Bug 423839 has been marked as a duplicate of this bug. ***