main/postmarketos-base-ui: add tinydm subpackage to pull in xinit (MR 4814)

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

main/postmarketos-base-ui/APKBUILD

 # Maintainer: Clayton Craft <clayton@craftyguy.net>
 pkgname=postmarketos-base-ui
 pkgver=18
-pkgrel=0
+pkgrel=1
 pkgdesc="Meta package for minimal postmarketOS UI base"
 url="https://postmarketos.org"
 arch="noarch"
@@ -31,6 +31,7 @@ subpackages="
 	$pkgname-qt-tweaks:qt_tweaks
 	$pkgname-qt-wayland:qt_wayland
 	$pkgname-tinydm
+	$pkgname-tinydm-x11:tinydm_x11
 	$pkgname-wifi-iwd:iwd
 	$pkgname-wifi-wpa_supplicant:wpa_supplicant
 	$pkgname-x11
@@ -147,6 +148,13 @@ tinydm() {
 	amove etc/tinydm.d/env-wayland.d/50-sdl-wayland.sh
 }
 
+tinydm_x11() {
+	install_if="$pkgname-tinydm=$pkgver-r$pkgrel xorg-server"
+	depends="xinit"
+
+	mkdir "$subpkgdir"
+}
+
 pulseaudio() {
 	install_if="$pkgname=$pkgver-r$pkgrel pulseaudio"
 	depends="pulseaudio-utils"