fix(pkg): declare gtk4 build deps for GTK rust apps

moongreet/moonlock/moonset link against gtk4 headers at build time.
The CI workflow runs `makepkg -sfd` and only installs makedepends
explicitly — with gtk4 listed only in depends, the build container
had no cairo.pc / gtk4.pc and cairo-sys-rs failed to find the
system library. Add gtk4, gtk4-layer-shell, pkgconf (plus
gtk-session-lock + pam for moonlock) to makedepends.

This previously only worked because repeated `makepkg -s` runs
left the libs installed in the act_runner container filesystem.
Recreating the runner today exposed the latent gap.

moongreet-git/.SRCINFO

@@ -8,6 +8,9 @@ pkgbase = moongreet-git
 	license = MIT
 	makedepends = git
 	makedepends = cargo
+	makedepends = gtk4
+	makedepends = gtk4-layer-shell
+	makedepends = pkgconf
 	depends = gtk4
 	depends = gtk4-layer-shell
 	depends = greetd