systemd: Track postinst/prerm in task checksum

When these functions change, the package should rebuild but currently it
does not. We need to add the dependencies manually as the dependency
code can't track dynamically created variables.

(From OE-Core rev: c5d7100a358244085a697a23790676df5eb4afa3)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/systemd.bbclass

@@ -48,6 +48,9 @@ if type systemctl >/dev/null 2>/dev/null; then
 fi
 }
 
+
+systemd_populate_packages[vardeps] += "systemd_prerm systemd_postinst"
+
 python systemd_populate_packages() {
     if "systemd" not in d.getVar("DISTRO_FEATURES", True).split():
         return