community/libtorrent-rasterbar: use c++17
libtorrent-rasterbar segfaults when using the default c++ standard (14) in 3.15. There is a upstream bug report [0] stating that using 17 fixes the crash, and I can confirm that is the case for me as well.
Reported upstream [1].
0: https://github.com/arvidn/libtorrent/issues/6028 1: https://github.com/arvidn/libtorrent/issues/6567