samples: bpf: refactor header include path

Currently, header inclusion in each file is inconsistent.
For example, "libbpf.h" header is included as multiple ways.

    #include "bpf/libbpf.h"
    #include "libbpf.h"

Due to commit b552d33c ("samples/bpf: fix include path
in Makefile"), $(srctree)/tools/lib/bpf/ path had been included
during build, path "bpf/" in header isn't necessary anymore.

This commit removes path "bpf/" in header inclusion.

Signed-off-by: Daniel T. Lee <danieltimlee@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>