cmake: Use precise 4-component SDK version for Windows SDK

CMake internally manages the Windows SDK version as a 4-component
version string. If only provided with 3 components, CMake will
implicitly add a "0" as the fourth component and the internal
CMAKE_VS_WINDOWS_TARGET_PLATFORM_VERSION variable will represent that
4-component string.

As this variable is used to set up sub-projects for other platforms,
re-creating them will fail if the original version is only provided
in its 3-component form.
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