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 b552d33c80a9 ("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>
diff --git a/samples/bpf/ibumad_user.c b/samples/bpf/ibumad_user.c
index 097d761..cb5a8f9 100644
--- a/samples/bpf/ibumad_user.c
+++ b/samples/bpf/ibumad_user.c
@@ -25,7 +25,7 @@
 
 #include "bpf_load.h"
 #include "bpf_util.h"
-#include "bpf/libbpf.h"
+#include "libbpf.h"
 
 static void dump_counts(int fd)
 {