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 b552d33c80 ("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>

samples/bpf/xdp_router_ipv4_user.c

@@ -24,7 +24,7 @@
 #include <sys/ioctl.h>
 #include <sys/syscall.h>
 #include "bpf_util.h"
-#include "bpf/libbpf.h"
+#include "libbpf.h"
 #include <sys/resource.h>
 #include <libgen.h>