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
8 jobs for !27766 with patches/3772 in 9 minutes and 12 seconds (queued for 4 seconds)
detached
Status Job ID Name Coverage
  Lint
passed lint #546497
docker-alpine x86_64

00:00:29

 
  Build
passed build-aarch64 #546502
docker-alpine ci-build aarch64

00:00:30

passed build-armhf #546504
docker-alpine ci-build armhf

00:08:41

passed build-armv7 #546503
docker-alpine ci-build armv7

00:00:30

passed build-ppc64le #546501
docker-alpine ci-build ppc64le

00:06:18

passed build-s390x #546500
docker-alpine ci-build s390x

00:05:35

passed build-x86 #546499
docker-alpine ci-build x86

00:00:24

passed build-x86_64 #546498
docker-alpine ci-build x86_64

00:03:14