patch 8.1.2131: MSVC tests fail

Problem:    MSVC tests fail.
Solution:   Replace backslashes with slashes.
diff --git a/src/version.c b/src/version.c
index a064648..2dc9f9b 100644
--- a/src/version.c
+++ b/src/version.c
@@ -754,6 +754,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2131,
+/**/
     2130,
 /**/
     2129,