bindings: rust: feature gate unreleased features

bindings: rust: feature gate unreleased features

`gpiod_line_request_get_chip_name()` is not released yet. Still, libgpiod-sys
will just happily generate bindings for it if it sees the definition in the
header file.

This guards the unreleased features behind an optional feature `vnext`.

To sketch the process of what happens once these features get into an
assumed "2.1" release:

libgpiod-sys will get updated with a `v2_1` feature. That feature would
then raise the minimum version that is attempted to query from pkg-
config. An identical feature will then be introduced on the `libgpiod`
crate and `vnext` guards will be changed to `v2_1` guards. The `vnext`
feature will then be updated to require the new `v2_1` feature.

Eventually, we will probably raise the minimum supported version for the
rust bindings and drop all the version gates before that.

Signed-off-by: Erik Schilling <erik.schilling@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>