patch 8.1.0589: compilation error in gvimext.cpp

Problem:    Compilation error in gvimext.cpp.
Solution:   Return a value.  Also fix using uninitialized variable.
diff --git a/src/version.c b/src/version.c
index 3c9eb0d..f868e6a 100644
--- a/src/version.c
+++ b/src/version.c
@@ -800,6 +800,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    589,
+/**/
     588,
 /**/
     587,