Fix iconv linking for MinGW builds

[Hop311]

This fixes MinGW GCC builds which were previously failing with:

[1/12] Linking CXX executable bin\Debug\openvic-dataloader.headless.exe
FAILED: [code=1] bin/Debug/openvic-dataloader.headless.exe 
C:\Windows\system32\cmd.exe /C "cd . && C:\msys64\mingw64\bin\c++.exe -g  CMakeFiles/openvic-dataloader-headless.dir/Debug/src/headless/main.cpp.obj -o bin\Debug\openvic-dataloader.headless.exe -Wl,--out-implib,Debug\libopenvic-dataloader.headless.dll.a -Wl,--major-image-version,0,--minor-image-version,0  bin/Debug/libopenvic-dataloader.a  _deps/fmt-build/Debug/libfmtd.a  _deps/vmcontainer-build/Debug/libvmcontainer-d.a  _deps/lexy-build/src/Debug/liblexy_file.a  -lkernel32 -luser32 -lgdi32 -lwinspool -lshell32 -lole32 -loleaut32 -luuid -lcomdlg32 -ladvapi32 && cd ."
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/16.1.0/../../../../x86_64-w64-mingw32/bin/ld.exe: bin/Debug/libopenvic-dataloader.a(Parser.cpp.obj): in function `auto ovdl::convert::gbk::_make_buffer<lexy::utf8_char_encoding, (lexy::encoding_endianness)2>::operator()<void>(ovdl::detail::Encoding, void const*, unsigned long long, void*) const':
S:/openvic/openvic/extension/deps/openvic-simulation/deps/openvic-dataloader/src/openvic-dataloader/detail/ConvertGbk.hpp:48:(.text$_ZNK4ovdl7convert3gbk12_make_bufferIN4lexy18utf8_char_encodingELNS3_19encoding_endiannessE2EEclIvEEDaNS_6detail8EncodingEPKvyPT_[_ZNK4ovdl7convert3gbk12_make_bufferIN4lexy18utf8_char_encodingELNS3_19encoding_endiannessE2EEclIvEEDaNS_6detail8EncodingEPKvyPT_]+0x78): undefined reference to `libiconv_open'
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/16.1.0/../../../../x86_64-w64-mingw32/bin/ld.exe: S:/openvic/openvic/extension/deps/openvic-simulation/deps/openvic-dataloader/src/openvic-dataloader/detail/ConvertGbk.hpp:213:(.text$_ZNK4ovdl7convert3gbk12_make_bufferIN4lexy18utf8_char_encodingELNS3_19encoding_endiannessE2EEclIvEEDaNS_6detail8EncodingEPKvyPT_[_ZNK4ovdl7convert3gbk12_make_bufferIN4lexy18utf8_char_encodingELNS3_19encoding_endiannessE2EEclIvEEDaNS_6detail8EncodingEPKvyPT_]+0x15c): undefined reference to `libiconv'
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/16.1.0/../../../../x86_64-w64-mingw32/bin/ld.exe: S:/openvic/openvic/extension/deps/openvic-simulation/deps/openvic-dataloader/src/openvic-dataloader/detail/ConvertGbk.hpp:219:(.text$_ZNK4ovdl7convert3gbk12_make_bufferIN4lexy18utf8_char_encodingELNS3_19encoding_endiannessE2EEclIvEEDaNS_6detail8EncodingEPKvyPT_[_ZNK4ovdl7convert3gbk12_make_bufferIN4lexy18utf8_char_encodingELNS3_19encoding_endiannessE2EEclIvEEDaNS_6detail8EncodingEPKvyPT_]+0x1a5): undefined reference to `libiconv_close'
collect2.exe: error: ld returned 1 exit status
[9/12] Building CXX object _deps/snitch-build/CMakeFiles/snitch.dir/Debug/src/snitch.cpp.obj
ninja: build stopped: subcommand failed.

This issue also impacted components higher up the dependency chain, preventing MinGW builds.