bpf: samples/sockmap fix Makefile for build error

While building samples/sockmap, undefined reference error is thrown
for `nla_dump_errormsg'.
Linking tools/lib/bpf/nlattr.o as a fix

Signed-off-by: Prashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>