patch 8.1.2366: using old C style comments

Problem:    Using old C style comments.
Solution:   Use // comments where appropriate.
diff --git a/src/iscygpty.h b/src/iscygpty.h
index 82fd0af..b053007 100644
--- a/src/iscygpty.h
+++ b/src/iscygpty.h
@@ -38,4 +38,4 @@
 #define is_cygpty_used()	0
 #endif
 
-#endif /* _ISCYGPTY_H */
+#endif // _ISCYGPTY_H