Drop bpftool version requirement to 4.18.0
The CentOS Stream 8 bpftool package has the skeleton generation
feature introduced in bpftool 5.6.0 backported to 4.18.0. The
meson script doesn't know this, so we downgrade the required
bpftool version to 4.18.0 to allow builds to pass on CentOS Stream
8.