bpf/selftests: test_select_reuseport_kern: Remove unused header

test_select_reuseport_kern.c is currently including <stdlib.h>, but it
does not use any definition from there.

Remove stdlib.h inclusion from test_select_reuseport_kern.c

Signed-off-by: Alexis Lothoré (eBPF Foundation) <[e-mail]>
Signed-off-by: Martin KaFai Lau <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/testing/selftests/bpf/progs/test_select_reuseport_kern.c

···
 // SPDX-License-Identifier: GPL-2.0
 /* Copyright (c) 2018 Facebook */
 
-#include <stdlib.h>
 #include <linux/in.h>
 #include <linux/ip.h>
 #include <linux/ipv6.h>