Remove deadcode warning in BuilderAttribs

In src/lib.rs remove the deadcode warning about the 'headless' builder
attribute. Headless is /actually/ set to false or true depending on if
HeadlessRendererBuilder is used.

src/lib.rs

@@ -154,6 +154,7 @@ pub struct WindowBuilder<'a> {
 
 /// Attributes
 struct BuilderAttribs<'a> {
+    #[allow(dead_code)]
     headless: bool,
     strict: bool,
     sharing: Option<&'a winimpl::Window>,