Fix build with setup.py on BSD

[abajk]

Building scapy with setup.py has been broken for some time. At least it does not work on FreeBSD. This commit fixes it.

Fixes: 669506b ("Migrate to pyproject.toml (#3869)")
Signed-off-by: Aleksander Jan Bajkowski olek2@wp.pl

[gpotter2]

Hi. Just to be sure, how exactly are you installing Scapy?

[abajk]

$ python3 setup.py --no-user-cfg  install --single-version-externally-managed --prefix="/usr" --root="<install dir>"

[abajk]

It's quite a complicated environment with cross compilation for another architecture. It worked up to version 2.5.0.
Full build log:

$ python3 setup.py --no-user-cfg  install --single-version-externally-managed --prefix="/usr" --root="<install dir>"
running install
running build
running install_data
creating <install dir>/usr
creating <install dir>/usr/share
creating <install dir>/usr/share/man
creating <install dir>/usr/share/man/man1
copying doc/scapy.1 -> <install dir>/usr/share/man/man1
running install_egg_info
running egg_info
writing UNKNOWN.egg-info/PKG-INFO
writing dependency_links to UNKNOWN.egg-info/dependency_links.txt
writing top-level names to UNKNOWN.egg-info/top_level.txt
reading manifest file 'UNKNOWN.egg-info/SOURCES.txt'
reading manifest template 'MANIFEST.in'
writing manifest file 'UNKNOWN.egg-info/SOURCES.txt'
Copying UNKNOWN.egg-info to <install dir>/usr/lib/python3.8/site-packages/UNKNOWN-0.0.0-py3.8.egg-info
running install_scripts