samples/bpf: fix include path

Fix the following build error:
HOSTCC  samples/bpf/test_lru_dist.o
../samples/bpf/test_lru_dist.c:25:22: fatal error: bpf_util.h: No such file or directory

This is due to objtree != srctree.
Use srctree, since that's where bpf_util.h is located.

Fixes: e00c7b21

 ("bpf: fix multiple issues in selftest suite and samples")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>