Using the windows.h
header guarantees correctness regardless of the content of other headers.
For more details, please refer to https://stackoverflow.com/questions/4845198/fatal-error-no-target-architecture-in-visual-studio
Fixes the MSVC build when using the upcoming CMake-based build system and Qt packages installed via the vcpkg package manager.
Related to https://github.com/hebasto/bitcoin/pull/77.