core: add the kernel uapi header to the repository

core: add the kernel uapi header to the repository

In order to avoid any problems with symbols missing from the host linux
kernel headers (for example: if current version of libgpiod supports
features that were added recently to the kernel but the host headers are
outdated and don't export required symbols) let's add the uapi header to
the repository and include it instead of the one in /usr/include/linux.

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