Required kernel interface was introduced in kernel v4.8,
so add the kernel version as dependency for this library.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>

libgpiod is a C library with corresponding tools for interacting
with the linux GPIO character device (gpiod stands for GPIO device).

Since linux 4.8 the GPIO sysfs interface is deprecated. User space should use
the character device instead. This library encapsulates the ioctl calls and
data structures behind a straightforward API.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>