fix: windows.h include on case-sensitive systems

[fruzitent]

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When cross compiling a Windows application with MinGW, there's a well-known problem with the missing "Windows.h" header due to filesystem case-sensitivity. Throughout the project uppercase variant occurs 13 times in 13 files and lowercase one - 55 times in 51 files. Even though only a single file needs to be changed for my needs, this commit ensures consistency by switching to lowercase everywhere.