Add unpacktool to CMake, port to *nix

- Adds missing includes, removes some unused ones.
- Removes unused Windows.h and shellapi.h includes.
- Uses / as path separator. This is portable and should still work fine
  on Windows.

unpacktool/IDecompressor.h

@@ -1,6 +1,6 @@
 #pragma once
 
-#include <stdint.h>
+#include <cstddef>
 
 struct CSInputBuffer;