The pinned topic has the solution
build() {
cd "${srcdir}/${_pkgname}-${_pkgver}/ebpf_prog"
KDIR="/usr/src/linux"
This is one of the reasons why using AUR on Manjaro is frowned upon.
It requires knowledge to workaround such issues.
Change KDIR to active kernel - e.g. linux68 - also be sure to have the headers installed for that kernel - which is not default.
Technically the maintainer could use dkms and thus avoid the whole debacle on where the headers are.