core: provide gpiod_line_request_get_chip_name()

core: provide gpiod_line_request_get_chip_name()

While we can get the list of requested offsets from a line-request object,
this information lacks context if we don't provide any data about the GPIO
chip the request was made on. Add a helper allowing users to get the name
of the parent chip.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>