patch 8.1.1575: callbacks may be garbage collected

Problem:    Callbacks may be garbage collected.
Solution:   Set reference in callbacks. (Ozaki Kiichi, closes #4564)
diff --git a/src/version.c b/src/version.c
index 41db458..019be44 100644
--- a/src/version.c
+++ b/src/version.c
@@ -778,6 +778,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1575,
+/**/
     1574,
 /**/
     1573,