mkinitcpio: fix permission of service overlay

When umask of root is set to e.g. 0077 and a mkinitcpio hook creates a overlay of a systemd unit with add_systemd_drop_in, the resulting file is not world-readable. Systemd then complains:

Configuration file /etc/systemd/system/$unit.d/$overlay.conf is marked world-inaccessible. This has no effect as configuration data is accessible via APIs without restrictions. Proceeding anyway.

This patch removes the systemd warning.