build: Install in /usr/lib instead of /usr/lib64 on Fedora

When building gnome-user-share in Fedora, gnome-user-share-webdav.service ends up incorrectly placed in /usr/lib64/systemd/user. It should be in /usr/lib/systemd/user.

This is caused by us building a path from scratch, so that the prefix could be overridden when doing an installation into a staging directory, as the path supplied in systemd's pkgconfig file was broken in this regard. This was however fixed in: https://github.com/systemd/systemd/issues/9595 so use this instead.

Closes: #22 (closed)