Fix LONG instead of LRESULT return in winuser.d

[dkorpel]

These functions are supposed to return a long on 64-bit, while a LONG is always 32-bit on Windows.

https://learn.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-dispatchmessagew
https://learn.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-senddlgitemmessagew

Found by LDC when using custom headers:

Error: Function type does not match previously declared function with the same mangled name: `DispatchMessageW`
        Previous IR type: i32 (ptr)
        New IR type:      i64 (ptr)

[dlang-bot]

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