phosphor-dbus-monitor: Add sdeventplus DEPEND

An upcoming change will require that phospohr-dbus-monitor have
sdeventplus available.

(From meta-phosphor rev: b27564d13d3ca404b1b39bdd4870234b440651ed)

Change-Id: I527daf07e623d18af4ddeecf0e99fa86465e4ba4
Signed-off-by: William A. Kennington III <wak@google.com>
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>