patch 9.0.0028: MS-Windows: tests fail if there is a "runtime" directory

Problem:    MS-Windows: tests fail if there is a stray "runtime" directory.
Solution:   Only use a "runtime" directory if it contains "defaults.vim".
diff --git a/src/version.c b/src/version.c
index 4585915..77be61c 100644
--- a/src/version.c
+++ b/src/version.c
@@ -736,6 +736,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    28,
+/**/
     27,
 /**/
     26,