patch 9.0.1560: Win32: When 'encoding' is set $PATH has duplicate entries

Problem:    Win32: When 'encoding' is set $PATH has duplicate entries.
Solution:   Only append the directory if it is not there yet. (Ken Takata,
            closes #12400, closes #12372)
diff --git a/src/version.c b/src/version.c
index 47079f5..7d691e3 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1560,
+/**/
     1559,
 /**/
     1558,