E0106: field lifetimes

I've added an example for custom type lifetimes located in another `struct` fields.

src/librustc/diagnostics.rs

@@ -362,6 +362,10 @@ Here are some simple examples of where you'll run into this error:
 struct Foo { x: &bool }        // error
 struct Foo<'a> { x: &'a bool } // correct
 
+struct Bar{ x: Foo }
+               ^^^ expected lifetime parameter
+struct Bar<'a>{ x: Foo<'a> } // correct
+
 enum Bar { A(u8), B(&bool), }        // error
 enum Bar<'a> { A(u8), B(&'a bool), } // correct