Add missing `#include` for macOS Catalina

[carlocab]

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.

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[vitaut]

Thank you!