Per @SoftFever review on #13752, third-party vendored libraries belong in
deps_src/ alongside expat, imgui, hidapi, etc.
- All 5 files (mdns.{h,c}, cxmdns.{h,cpp}, NOTICE.md) move from
src/slic3r/Utils/mdns/ to deps_src/mdns/.
- deps_src/mdns/CMakeLists.txt builds mdns as a STATIC library and scopes
the MSVC Iphlpapi/Ws2_32 link requirement to that target instead of
libslic3r_gui's global MSVC block.
- deps_src/CMakeLists.txt gains add_subdirectory(mdns).
- src/slic3r/CMakeLists.txt drops the inline source listings and links
libslic3r_gui against the new mdns target; MSVC block keeps only
Setupapi.lib.
- src/slic3r/Utils/CrealityHostDiscovery.cpp #include updated to use the
include dir exposed by the new mdns target.
Verified by a clean Linux build (orca-slicer links successfully).
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