SwiftWin32: compensate for GetComputerNameW's disregard of the null terminator

[relgit]

Hi @compnerd, before I jump to the PR I just wanted to say thanks for all the work you're putting into this project 😀

Issue

GetComputerNameW disregards the null terminator when returning the size of the buffer. This leads to the following error when using Device.name:

Swift/CString.swift:284: Fatal error: input of decodeCString(_:as:repairingInvalidCodeUnits:) must be null-terminated

Solution

Compensated the buffer size to account for the null terminator.

[compnerd]

Thank you for your contribution!