missing definition for MSYS builds

[phlptp]

In file included from C:/Users/phlpt/Documents/HELICS_MAIN/ThirdParty/asio/asio/include/asio/io_context.hpp:18,
from C:/Users/phlpt/Documents/HELICS_MAIN/ThirdParty/networking/gmlc/networking/AsioContextManager.h:22,
from C:/Users/phlpt/Documents/HELICS_MAIN/ThirdParty/networking/gmlc/networking/AsioContextManager.cpp:19:
C:/Users/phlpt/Documents/HELICS_MAIN/ThirdParty/asio/asio/include/asio/detail/config.hpp:1383:5: warning: #warning Please define _WIN32_WINNT or _WIN32_WINDOWS appropriately. [-Wcpp]
1383 | # warning Please define _WIN32_WINNT or _WIN32_WINDOWS appropriately.
| ^~~~~~~

need to add a _WIN32_WINNT definition for MSYS builds