postmarketos-ui-xfce4: create symlink at old pmOS icon location (MR 2916)

The postmarketOS icon was moved to a new location (see issue
#1418). Therefore the button of whiskermenu in the panel
becomes empty. This commit creates a symlink at the old location
to restore the icon on existing Xfce4 installations.

[ci:skip-build] already built successfully in CI

main/postmarketos-ui-xfce4/APKBUILD

@@ -2,7 +2,7 @@
 # Maintainer: Jakko <jahau@rocketmail.com>
 pkgname=postmarketos-ui-xfce4
 pkgver=0.5.0
-pkgrel=0
+pkgrel=1
 pkgdesc="(X11) Lightweight desktop (stylus recommended)"
 url="https://gitlab.com/postmarketOS/xfce4-phone"
 arch="noarch"
@@ -62,6 +62,10 @@ package() {
 
 	install -Dm644 "$srcdir"/onboard-autostart.desktop \
 		"$pkgdir"/etc/skel/.config/autostart/onboard-autostart.desktop
+
+	# Issue pmaports#1418, can be removed after v21.12 became unsupported
+	mkdir -p "$pkgdir"/usr/share/icons/postmarketOS
+	ln -s /usr/share/pixmaps/postmarketos-logo.svg "$pkgdir"/usr/share/icons/postmarketOS/logo.svg
 }
 
 sha512sums="