Add include that is needed in msys2

Summary: In the latest msys2, they updated some windows headers to require this extra include. It's not specific to this project, openFrameworks had to do it as well.

Test Plan: None, it just fixes a build. If I could get some guidance I would be willing to try to get a CI build for it.

Reviewers: Frameworks, kfunk

Reviewed By: kfunk

Tags: Frameworks, Windows

Differential Revision: https://phabricator.kde.org/D4778