harktech: move vendored mdns into deps_src/ per repo convention

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).

src/slic3r/Utils/CrealityHostDiscovery.cpp

@@ -1,5 +1,5 @@
 #include "CrealityHostDiscovery.hpp"
-#include "mdns/cxmdns.h"
+#include "cxmdns.h"
 #include "Http.hpp"
 
 #include <boost/log/trivial.hpp>