tinydm-run-session: parse .desktop arrays correctly

$ cat ../phosh/data/wayland-sessions/phosh.desktop 
[Desktop Entry]
Name=Phosh
Comment=Phone Shell
Comment=This session logs you into Phosh
Exec=phosh-session
Type=Application
DesktopNames=Phosh;GNOME;
$ parse_xdg_desktop ../phosh/data/wayland-sessions/phosh.desktop "DesktopNames"
Phosh:GNOME

Citing 1:

$XDG_CURRENT_DESKTOP should have been set by the login manager, according to the value of the DesktopNames found in the session file. The entry in the session file has multiple values separated in the usual way: with a semicolon.

Fixes: e7a35226

Those ;-separted values is specified in https://specifications.freedesktop.org/desktop-entry-spec/latest/value-types.html