Fix tcpdump detection

[gpotter2]

fixes #2223

[guedou]

Please don’t merge until tested on OpenBSD.

[codecov]

Codecov Report

Merging #2225 into master will decrease coverage by 1.2%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2225      +/-   ##
==========================================
- Coverage   84.78%   83.57%   -1.21%     
==========================================
  Files         202      207       +5     
  Lines       46098    46613     +515     
==========================================
- Hits        39082    38956     -126     
- Misses       7016     7657     +641

Impacted Files	Coverage Δ
scapy/arch/common.py	66.66% <100%> (-23.34%)	⬇️
scapy/arch/bpf/supersocket.py	27.77% <0%> (-50.01%)	⬇️
scapy/arch/linux.py	53.68% <0%> (-23.76%)	⬇️
scapy/layers/dhcp6.py	64.18% <0%> (-21.56%)	⬇️
scapy/compat.py	79.22% <0%> (-20.78%)	⬇️
scapy/supersocket.py	53.4% <0%> (-18.19%)	⬇️
scapy/layers/l2.py	76.02% <0%> (-9.45%)	⬇️
scapy/sendrecv.py	76.87% <0%> (-6.03%)	⬇️
scapy/arch/bpf/core.py	81.37% <0%> (-5.89%)	⬇️
... and 17 more

[p-l-]

This works (as it is, and with the suggestion) on OpenBSD 6.5.