nfs: Fix incorrect engine registering for '--enghelp' list

`ioengine` from `nfs` (internal) engine is incorrectly exported thus
overriding its value in constructor callbacks of other external engines,
that are used for registering engine for listing with `--enghelp`.

Because flist is unsafe to double adding it also making `engine_list` to
become corrupt and causing infinite loop or abnormal list termination
when printing engine list.

Issue: #1655
Fixes: 9326926 ("NFS engine")
Signed-off-by: Vitaly Chikunov <vt@altlinux.org>

engines/nfs.c

@@ -308,7 +308,7 @@ static int fio_libnfs_close(struct thread_data *td, struct fio_file *f)
 	return ret;
 }
 
-struct ioengine_ops ioengine = {
+static struct ioengine_ops ioengine = {
 	.name		= "nfs",
 	.version	= FIO_IOOPS_VERSION,
 	.setup		= fio_libnfs_setup,