Fix build error on UWP (#896)

For x64-uwp, size_t is 64-bit, DWORD is 32-bit.
When /WX compiler option is specified, error occurs.

windows/toni_ronnko_dirent.h

@@ -339,7 +339,13 @@ static void dirent_set_errno(int error);
  */
 static _WDIR *_wopendir(const wchar_t *dirname) {
   _WDIR *dirp;
+#if WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_DESKTOP)
+  /* Desktop */
   DWORD n;
+#else
+  /* WinRT */
+  size_t n;
+#endif
   wchar_t *p;
 
   /* Must have directory name */