patch 8.0.0558: :ownsyntax is not tested

Problem:    The :ownsyntax command is not tested.
Solution:   Add a test. (Dominique Pelle, closes #1622)
diff --git a/src/version.c b/src/version.c
index d37033f..26df634 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    558,
+/**/
     557,
 /**/
     556,