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).

deps_src/mdns/CMakeLists.txt

@@ -0,0 +1,19 @@
+cmake_minimum_required(VERSION 3.13)
+
+project(mdns)
+
+# Static library wrapping mjansson/mdns (public domain) plus the cxmdns C++
+# wrapper from CrealityPrint v7.1.1 (AGPL-3.0). See NOTICE.md for attribution.
+add_library(mdns STATIC
+    mdns.h
+    mdns.c
+    cxmdns.h
+    cxmdns.cpp
+)
+
+target_include_directories(mdns SYSTEM PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})
+
+if (MSVC)
+    # mjansson/mdns uses GetAdaptersAddresses (Iphlpapi) and Winsock2 (Ws2_32).
+    target_link_libraries(mdns PUBLIC Iphlpapi Ws2_32)
+endif()