Dungeongen: Also preserve river water nodes

For future river mapgens
Dungeons will not generate in river water, to
avoid dungeons filling and blocking river channels

src/dungeongen.h

@@ -40,6 +40,7 @@ int dir_to_facedir(v3s16 d);
 
 struct DungeonParams {
 	content_t c_water;
+	content_t c_river_water;
 	content_t c_cobble;
 	content_t c_moss;
 	content_t c_stair;