This reverts commit 55bf6b50.

This accidentally removed things that should have stayed - libseat
can still use the logind API, even if weston doesn't directly use
it.

Note that the logind-launcher does not actually build anymore
because breaking changes landed before this revert.

Since we're removing it again right away, I've not taken care to
fix that.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>