package/libiio: bump to version 0.15

package/libiio: bump to version 0.15

This change bumps libiio, to version 0.15.
This version is currently the most stable version in the series. It
contains several fixes over 0.14.

0.16 & 0.17 have been released but they have some issues with backwards
compatibility, so they are not yet recommended.

Changelog for version 0.15 (over 0.14).
Link:
  https://github.com/analogdevicesinc/libiio/releases/tag/v0.15

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>