package/shairport-sync: bump to version 2.1.8

Bump to version 2.1.8:
* remove comment about COPYING (fixed upstream)
* adapt ssl configure option for openssl and polarssl

Signed-off-by: Jörg Krause <jkrause@posteo.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>