libsa: mark head_errlog feature supported.

head_errlog is new format for errlog, but we do not really
use errlog, so we can just mark it supported, to enable reading
from pool.

MFC after:	1 week

stand/libsa/zfs/zfsimpl.c

@@ -139,6 +139,7 @@ static const char *features_for_read[] = {
 	"com.intel:allocation_classes",
 	"org.freebsd:zstd_compress",
 	"com.delphix:bookmark_written",
+	"com.delphix:head_errlog",
 	NULL
 };