Even though many of those scripts are very simple, it is easier to include
the header than to try to say whether each of those files is trivial enough
not to require one.
The script is renamed to match.
Now all targets are named uniformly in a tab-completion-friendly fashion, with
the exception of systemd-update-po which is generated by the i18n module
automatically:
$ ninja -C build -t targets | grep update
systemd-update-po: phony
update-syscall-tables: phony
update-syscall-header: phony
update-hwdb: phony
update-hwdb-autosuspend: phony
update-dbus-docs: CUSTOM_COMMAND
update-man-rules: CUSTOM_COMMAND