patch 8.1.2396: using old C style comments

Problem:    Using old C style comments.
Solution:   Use // comments where appropriate.
diff --git a/src/xpm_w32.c b/src/xpm_w32.c
index 20612c5..111988a 100644
--- a/src/xpm_w32.c
+++ b/src/xpm_w32.c
@@ -16,7 +16,7 @@
 
 #include "xpm_w32.h"
 
-/* Engage Windows support in libXpm */
+// Engage Windows support in libXpm
 #define FOR_MSW
 
 #include "xpm.h"
@@ -33,8 +33,8 @@
     HBITMAP *hImage,
     HBITMAP *hShape)
 {
-    XImage	    *img;  /* loaded image */
-    XImage	    *shp;  /* shapeimage */
+    XImage	    *img;  // loaded image
+    XImage	    *shp;  // shapeimage
     XpmAttributes   attr;
     int		    res;
     HDC		    hdc = CreateCompatibleDC(NULL);