Add an optional dependency on DevTest's mapgen mod (#176)

Author: Watilin

mod.conf

@@ -1,5 +1,5 @@
 name = pipeworks
 description = This mod uses mesh nodes and nodeboxes to supply a complete set of 3D pipes and tubes, along with devices that work with them.
 depends = basic_materials, xcompat, fakelib
-optional_depends = mesecons, mesecons_mvps, digilines, signs_lib, unified_inventory, default, screwdriver, fl_mapgen, sound_api, i3, hades_core, hades_furnaces, hades_chests, mcl_mapgen_core, mcl_barrels, mcl_furnaces, mcl_experience, vizlib
+optional_depends = mesecons, mesecons_mvps, digilines, signs_lib, unified_inventory, default, screwdriver, fl_mapgen, sound_api, i3, hades_core, hades_furnaces, hades_chests, mcl_mapgen_core, mcl_barrels, mcl_furnaces, mcl_experience, vizlib, mapgen
 min_minetest_version = 5.5.0