patch 8.0.0555: toupper/tolower test fails on OSX without Darwin

Problem:    Toupper/tolower test fails on OSX without Darwin.
Solution:   Skip that part of the test also for OSX. (Kazunobu Kuriyama)
diff --git a/src/version.c b/src/version.c
index 578cfe3..7cf7644 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    555,
+/**/
     554,
 /**/
     553,