Add NewGRF feature: Extra station name strings

These are used when all the default names have been used up,
instead of "Town Station #NNN".

src/saveload/station_sl.cpp

@@ -449,6 +449,7 @@ static const SaveLoad _station_desc[] = {
 	  SLE_CONDREF(Station, airport.psa,                REF_STORAGE,               SLV_161, SL_MAX_VERSION),
 
 	      SLE_VAR(Station, indtype,                    SLE_UINT8),
+	SLE_CONDVAR_X(Station, extra_name_index,           SLE_UINT16,          SL_MIN_VERSION, SL_MAX_VERSION, SlXvFeatureTest(XSLFTO_AND, XSLFI_EXTRA_STATION_NAMES)),
 
 	      SLE_VAR(Station, time_since_load,            SLE_UINT8),
 	      SLE_VAR(Station, time_since_unload,          SLE_UINT8),