- fix tests on OpenBSD which requires _DYNAMIC symbol

Authored by kgardas.

Description

  • fix tests on OpenBSD which requires _DYNAMIC symbol

Summary:
This patch adds _DYNAMIC symbol to the list of OpenBSD symbols.
The patch fixes unknown _DYNAMIC symbol runtime linker error caused
by recent update of unix library.

Reviewers: bgamari, austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1689