gl/sys/gir: New buffer/memory get_type functions are only in 1.20
G-IR cannot read or represent Since: annotations on get_type()-like
functions, despite these being available in the source. This adds the
type requirements manually following its implementation in 1,
considering it will not be backported to 1.18.x 2. If get_type is
used in manual bindings after 1.20 releases this version requirement is
easily caught.
In hindsight, while nice to see in the source, this might not be a hard requirement as it should be caught if/when the CI links against <1.20 libraries?