main/postmarketos-base-ui: add tinydm subpackage to pull in xinit
tinydm uses startx for Xorg UIs, which is provided by the xinit package. The xinit package used to be pulled in automatically by the xorg-server package, but this stopped being the case with https://gitlab.alpinelinux.org/alpine/aports/-/commit/1fd2d70696b88a0605ad5771137da148c998c192