patch 8.1.2366: using old C style comments

Problem:    Using old C style comments.
Solution:   Use // comments where appropriate.
diff --git a/src/gui_x11_pm.h b/src/gui_x11_pm.h
index 24cd2ca..e327bdb 100644
--- a/src/gui_x11_pm.h
+++ b/src/gui_x11_pm.h
@@ -84,7 +84,7 @@
     tb_exit_xpm
 };
 
-/* Indices for named colors */
+// Indices for named colors
 #define BACKGROUND	0
 #define FOREGROUND	1
 #define BOTTOM_SHADOW	2