Support BoxedInline without a get_type function

[bilelmoussaoui]

currently gir panics when it encounters a type that doesn't have a get_type function

thread 'main' panicked at 'Record ToplevelSize has record_boxed=true but don't have glib:get_type function', src/codegen/record.rs:31:13

cc @sdroege

[bilelmoussaoui]

Closing as a duplicate of #1297