Add missing `#include` for macOS Catalina (#517)
Add missing `#include` for macOS Catalina (#517)

Summary:
Without this, the build fails on macOS Catalina with the error

    error: use of undeclared identifier 'errno'
        errno = 0;
        ^

Notably, this does not happen on macOS Big Sur, Monterey, or on Linux.
Seen while building the latest tag of fbthrift for Homebrew at
Homebrew/homebrew-core#108626.

Build logs available at https://github.com/Homebrew/homebrew-core/runs/7955648533?check_suite_focus=true#step:6:281.

Pull Request resolved: https://github.com/facebook/fbthrift/pull/517

Reviewed By: roddym

Differential Revision: D38941391

Pulled By: vitaut

fbshipit-source-id: aeef64532ac29cece5d8e9f8822cac108a9e2b4f

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