samples: bpf: compile and link against full libbpf

samples/bpf currently cherry-picks object files from tools/lib/bpf
to link against.  Just compile the full library and link statically
against it.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>