Since libbpf now links against zlib, this needs to be included in the
linker invocation for the userspace programs in samples/bpf that link
statically against libbpf.

Fixes: 166750bc ("libbpf: Support libbpf-provided extern variables")
Signed-off-by: NToke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: NAlexei Starovoitov <ast@kernel.org>
Tested-by: NBjörn Töpel <bjorn.topel@intel.com>
Link: https://lore.kernel.org/bpf/20191216102405.353834-1-toke@redhat.com