net/libdnet: disable useless buildtime check for /dev/bpf presence

Device is not guarateed to exist at package building time.

PR: 238523
Submitted by: rea@FreeBSD.org
Reported by: dweimer@dweimer.net
Approved by: maintainer timeout (onatan@gmail.com, 1.5 years)
MFH: 2021Q1