Add initial support for loading JokerPP savegames

See #123

src/saveload/economy_sl.cpp

@@ -46,6 +46,7 @@ static const SaveLoad _economy_desc[] = {
 	    SLE_VAR(Economy, infl_amount,                   SLE_UINT8),
 	    SLE_VAR(Economy, infl_amount_pr,                SLE_UINT8),
 	SLE_CONDVAR(Economy, industry_daily_change_counter, SLE_UINT32,                SLV_102, SL_MAX_VERSION),
+	SLE_CONDNULL_X(8, SL_MIN_VERSION, SL_MAX_VERSION, SlXvFeatureTest(XSLFTO_AND, XSLFI_JOKERPP)),
 	    SLE_END()
 };