install/sd-vconsole: include the us keymap if KEYMAP is not set

nl6720 requested to merge nl6720/mkinitcpio:sd-vconsole-us into master

If KEYMAP is not set in /etc/vconsole.conf, systemd-vconsole-setup will fall back to the "us" keymap.

Ensure a keymap file is included in initramfs to prevent systemd-vconsole-setup.service from failing.

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