patch 8.1.2366: using old C style comments

Problem:    Using old C style comments.
Solution:   Use // comments where appropriate.
diff --git a/src/os_qnx.h b/src/os_qnx.h
index f98e311..503805a 100644
--- a/src/os_qnx.h
+++ b/src/os_qnx.h
@@ -12,7 +12,7 @@
 
 #define	USE_TMPNAM
 
-#define POSIX	    /* Used by pty.c */
+#define POSIX	    // Used by pty.c
 
 #if defined(FEAT_GUI_PHOTON)
 extern int is_photon_available;