iio: Export userspace IIO headers

After UAPI header file split [1] all user-kernel interfaces were
placed under include/uapi/.

This patch moves IIO user specific API from:
	* include/linux/iio/events.h => include/uapi/linux/iio/events.h
	* include/linux/types.h => include/uapi/linux/types.h

Now there is no need for nasty tricks to compile userspace programs
(e.g iio_event_monitor). Just installing the kernel headers with
make headers_install command does the job.

[1] http://lwn.net/Articles/507794/

Signed-off-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>