meson: look for bpftool in /usr/sbin too

On Debian, bpftool is installed in /usr/sbin, which is not in $PATH for
non-root users by default, so finding it fails.
Add a secondary, hard-coded '/usr/sbin/bpftool' after 'bpftool' so that
meson can find it.

https://packages.debian.org/sid/amd64/bpftool/filelist