Add NewGRF object property for flood-resistance

Does not imply being able to be built on water

src/newgrf_extension.h

@@ -34,6 +34,7 @@ enum Action0RemapPropertyIds {
 	A0RPI_SIGNALS_EXTRA_ASPECTS,
 	A0RPI_OBJECT_USE_LAND_GROUND,
 	A0RPI_OBJECT_EDGE_FOUNDATION_MODE,
+	A0RPI_OBJECT_FLOOD_RESISTANT,
 };