package/kodi-inputstream-adaptive: bump version to 2.6.15-Matrix

Changelog:
https://github.com/xbmc/inputstream.adaptive/blob/Matrix/inputstream.adaptive/addon.xml.in#L22



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>