bpf: Remove unnecessary <argp.h> include from preload/iterators

This program does not use argp (which is a glibcism). Instead include <errno.h>
directly, which was pulled in by <argp.h>.
Signed-off-by: Leah Neukirchen <leah@vuxu.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20201216100306.30942-1-leah@vuxu.orgSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/preload/iterators/iterators.c

 // SPDX-License-Identifier: GPL-2.0
 /* Copyright (c) 2020 Facebook */
-#include <argp.h>
+#include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>