glib: Make <Variant as HasParamSpec>::BuilderFn generic over lifetimes

[kawadakk]

This makes it possible to pass a non-'static &VariantTy to ParamSpecVariant::builder in #[derive(Properties)]. Example:

#[derive(Properties)]
#[properties(wrapper_type = super::Foo)]
pub struct Foo {
    #[property(get, set, builder(&<Option<i32>>::static_variant_type()))]
    variant: RefCell<Option<glib::Variant>>,
}