Go: Add missing +build and go:build comments

A RHEL 8 test was failing because it uses go1.16. The old style must
be retained for backwards compat.

Fixes: 9a36de84c ("Go: Use Homebrew include paths")
Reviewed-by: Andrew Clayton <a.clayton@nginx.com>
Reviewed-by: Dylan Arbour <d.arbour@f5.com>
Signed-off-by: Danielle De Leo <d.deleo@f5.com>

go/ldflags-darwin.go

@@ -1,4 +1,5 @@
 //go:build darwin
+// +build darwin
 
 /*
  * Copyright (C) Danielle De Leo