Add an extended version feature for map sizes > 8kx8k.

This is to prevent earlier versions from trying to load the savegame
and aborting.

src/saveload/extended_ver_sl.h

@@ -21,6 +21,7 @@
  */
 enum SlXvFeatureIndex {
 	XSLFI_NULL                          = 0,      ///< Unused value, to indicate that no extended feature test is in use
+	XSLFI_EXTRA_LARGE_MAP,                        ///< Extra large map
 
 	XSLFI_SIZE,                                   ///< Total count of features, including null feature
 };