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

[Albert Mingkun Yang]

Trivial fix of macro name.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/17446/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17446&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8323800
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/17446.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17446/head:pull/17446

PR: https://git.openjdk.org/jdk/pull/17446