If neither of __FreeBSD__, __FreeBSD_kernel__ and __DragonFly__ is defined,
util.h is included from tap-bsd.c.
Don't include it again if __OpenBSD__ is defined.

Cc: Blue Swirl <blauwirbel@gmail.com>
Signed-off-by: Andreas Färber <andreas.faerber@web.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>