Add fast_float to the include dir

[jackylee-ch]

If we want use fast_float in sub libraries, such as gazelle, we need add it to the include dir.

[github-actions]

Thanks for opening a pull request!

If this is not a minor PR. Could you open an issue for this pull request on JIRA? https://issues.apache.org/jira/browse/ARROW

Opening JIRAs ahead of time contributes to the Openness of the Apache Arrow project.

Then could you also rename pull request title in the following format?

ARROW-${JIRA_ID}: [${COMPONENT}] ${SUMMARY}

or

MINOR: [${COMPONENT}] ${SUMMARY}

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• Other pull requests
• Contribution Guidelines - How to contribute patches