ICU-23142 Fix tiny header guarder in `icu4c/source/i18n/regexcxt.h`

[anonymouspc]

Header icu4c/source/i18n/regexcxt.h is wrongly guarded by RBBIRPT_H. I made it a tiny change to REGEXCST_H.

Recently I'm trying to build icu into a C++20 module, and we need a correct guardian in order to build across multiple module partitions. Thank you!

This is an amazing library an I will star it.

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Hooray! The files in the branch are the same across the force-push. 😃

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[jira-pull-request-webhook[bot]]

Hooray! The files in the branch are the same across the force-push. 😃

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