Improve Makefile for systemd target auto-completion

Explicitly declare individual systemd-related targets in the Makefile
to enable shell auto-completion functionality when using the `make`
command. This change replaces the previous pattern rule that used a wildcard
(%_systemd), which did not support auto-completion.

Now, running `make` followed by pressing the `<TAB>` key will suggest
all systemd-related targets like install_systemd, enable_systemd,
disable_systemd, and uninstall_systemd.

This enhances the developer experience by providing quick access to the
Makefile targets without having to manually type the full names or refer
back to the documentation.

Makefile

@@ -90,7 +90,7 @@ uninstall_graftcp:
 
 .PHONY: install_systemd enable_systemd disable_systemd uninstall_systemd
 
-%_systemd:
+install_systemd enable_systemd disable_systemd uninstall_systemd:
 	$(MAKE) -C local $@
 
 install_graftcp_local: