core: hide the GPIOD_API symbol

GPIOD_API is unneccesarily polluting the user-facing headers. There's no
advantage to having it in public files. Let's make them internal to the
library. Upcoming modifications will also make GPIOD_BIT() redundant so
we'll be able to remove all macros unrelated to the main functionality
from gpiod.h.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
13 files changed