phosphor-fan: Depend on sdeventplus

This will be needed for an upcoming change which causes
phosphor-fan-presence to require sdeventplus.

(From meta-phosphor rev: 581d3c2493b0aa94895ad7c74ce9994b09d52411)

Change-Id: I8ec0a768b6d9a06710515b21c5c312a071bd1dd2
Signed-off-by: William A. Kennington III <wak@google.com>
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>