dropbear: add dropbearkey.service to FILES

dropbear: add dropbearkey.service to FILES

dropbear.socket file contains:
  Also=dropbearkey.service

That's why dropbearkey.service got automatically added to FILES.
Since this incorrect auto adding feature gets dropped from
systemd.bbclass this needs to be added explicitly.

Signed-off-by: Adrian Freihofer <adrian.freihofer@siemens.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>