systemd: bpf-framework: 'propagate' the '--sysroot=' for crosscompilation

systemd: bpf-framework: 'propagate' the '--sysroot=' for crosscompilation

The eBPFs are pre-compiled during the systemd-build with a different
compiler than the cross-compiler used to build systemd itself.

This is either a 'clang-native' or a gcc (bpf-unknown-none) which do
not see the BUILD_CFLAGS, that point to the correct include search
patch. To address this have systemd's meson.build "propagate" the
--system from the C_FLAGS into the BPF compiler call.

Signed-off-by: Johannes Schneider <johannes.schneider@leica-geosystems.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>