libbpf: Use local bpf_helpers.h include

Commit 20d59ee55172fdf6 ("libbpf: add bpf_core_cast() macro") added a
bpf_helpers include in bpf_core_read.h as a system include. Usually, the
includes are local, though, like in bpf_tracing.h. This commit adjusts
the include to be local as well.

Signed-off-by: Tobias Böhm <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/q5d5bgc6vty2fmaazd5e73efd6f5bhiru2le6fxn43vkw45bls@fhlw2s5ootdb

tools/lib/bpf/bpf_core_read.h

···
 #ifndef __BPF_CORE_READ_H__
 #define __BPF_CORE_READ_H__
 
-#include <bpf/bpf_helpers.h>
+#include "bpf_helpers.h"
 
 /*
  * enum bpf_field_info_kind is passed as a second argument into