Fix compilation error on CentOS 8 (kernel-4.18.0-227)

Error message:

  ipt_NETFLOW.c: In function ‘ipt_netflow_fini’:
  ipt_NETFLOW.c:5789:2: error: implicit declaration of function ‘synchronize_sched’; did you mean ‘synchronize_net’? [-Werror=implicit-function-declaration]
    synchronize_sched();
    ^~~~~~~~~~~~~~~~~
    synchronize_net
  cc1: some warnings being treated as errors

Author: aabc

gen_compat_def

@@ -77,6 +77,8 @@ kbuild_test_struct timeval linux/ktime.h
 # 97a32539b9568 proc: convert everything to "struct proc_ops"
 # d56c0d45f0e27 proc: decouple proc from VFS with "struct proc_ops"
 kbuild_test_struct proc_ops linux/proc_fs.h
+# No since v5.1, but present in CentOS-8's 4.18.0-227
+kbuild_test_symbol synchronize_sched linux/rcupdate.h
 
 echo "// End of compat_def.h"
 

ipt_NETFLOW.c

@@ -5785,7 +5785,7 @@ static void __exit ipt_netflow_fini(void)
 	netflow_scan_and_export(AND_FLUSH);
 	del_timer_sync(&rate_timer);
 
-#if LINUX_VERSION_CODE < KERNEL_VERSION(5,1,0)
+#ifdef HAVE_SYNCHRONIZE_SCHED
 	synchronize_sched();
 #else
 	synchronize_rcu();