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>

samples/sockmap/Makefile

@@ -2,7 +2,7 @@
 hostprogs-y := sockmap
 
 # Libbpf dependencies
-LIBBPF := ../../tools/lib/bpf/bpf.o
+LIBBPF := ../../tools/lib/bpf/bpf.o ../../tools/lib/bpf/nlattr.o
 
 HOSTCFLAGS += -I$(objtree)/usr/include
 HOSTCFLAGS += -I$(srctree)/tools/lib/