Add Requires=buxton2.socket dependency to buxton2.service

Add Requires=buxton2.socket dependency to buxton2.service

In emergency mode, buxton2.service runs without buxton2.socket.
This patch prevents buxton2.service from running when buxton2.socket is not running.

In emergency mode, systemd does not start units with an implicit 'Requires=sysinit.target' dependency.
Because buxton2.socket does not have a 'DefaultDependency=no', it implicitly has 'Requires=sysinit.target'.
(Refer: https://www.freedesktop.org/software/systemd/man/systemd.socket.html)

Change-Id: I5ed9765b404012a4bb6036e7779f9c66d5ac5571