main/postmarketos-base-ui: add Xwrapper script for X11

Xorg 21.1.2 needs an Xwrapper.config script with the line needs_root_rights=yes to get tinydm + X11 UI to work.
xorg-server seemed to be broken on my devices (see !2605 (closed)) but fortunately, since 21.1.1, it was fixed and it only needs this small script to work with tinydm + any X11 UI (such as SXMO or LXQt) now.

Fixes the following:

Fatal server error:
(EE) xf86OpenConsole: Cannot open virtual console 7 (Permission denied)

Closes: #1150 (closed), #1236 (closed), !2605 (closed).
Related: aports#11347

Please check !2605 (closed) for more information.