sideload-repos-systemd: Dist service unconditionally

Add flatpak-sideload-usb-repo.service.in to EXTRA_DIST regardless of if
the --enable-auto-sideloading configure option was passed. This allows
building a tarball without that option and then building from the
tarball with the option.

This matches what is done in system-helper/Makefile.am.inc with
flatpak-system-helper.service.in.

Picked from https://github.com/endlessm/flatpak/pull/228

(cherry picked from commit f1383c10be)
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