RFR: 8323800: Serial: Fix include guard macro in generation.hpp

[Stefan Karlsson]

On Tue, 16 Jan 2024 12:06:39 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> Trivial fix of macro name.

Marked as reviewed by stefank (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/17446#pullrequestreview-1823224740