app: Enable building on OpenBSD

Currently the golang.org/x/sys package is missing the Pipe2 call for OpenBSD.
The call exists on OpenBSD, it just isn't exposed.

This diff was tested buy adding the Pipe2 call and setting:

   go mod edit -replace=golang.org/x/sys=/pat/to/modified/sys

Signed-off-by: Aaron Bieber <aaron@bolddaemon.com>

app/headless/headless_egl.go

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: Unlicense OR MIT
 
-// +build linux freebsd windows
+// +build linux freebsd windows openbsd
 
 package headless