systemd: add PACKAGECONFIG for bpf-framework

systemd: add PACKAGECONFIG for bpf-framework

The bpf-framework is used to pre-compile eBPFs that required for the
systemd.resource-control features RestrictFileSystems=[1] and
RestrictNetworkInterfaces=[2] to work.

Apart from 'clang-native' to compile the eBPFs, the required kernel
switches are described in [3].

Link: https://www.freedesktop.org/software/systemd/man/latest/systemd.resource-control.html#RestrictFileSystems=
Link: https://www.freedesktop.org/software/systemd/man/latest/systemd.resource-control.html#RestrictNetworkInterfaces=
Link: https://kinvolk.io/blog/2021/04/extending-systemd-security-features-with-ebpf/
Signed-off-by: Johannes Schneider <johannes.schneider@leica-geosystems.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>