Avoid const warning on Win32

scan_dir() alters the directory name in place on Win32 to convert \ to
/, so it can't be treated as const.

pkg.c

@@ -127,7 +127,7 @@ name_ends_in_uninstalled (const char *str)
  * locations, ignoring duplicates
  */
 static void
-scan_dir (const char *dirname)
+scan_dir (char *dirname)
 {
   DIR *dir;
   struct dirent *dent;