Update ktorrent to latest upstream release, 5.1.2
- also chase attendant libktorrent
- switch libktorrent to release tarballs instead of GitHub
From the release announcement, Fixes:
- Fix HTTP seeding.
- Fix memory corruption in ScanFolder plugin.
- Fix a few issues found by clang-tidy.
- Some build fixes (missing include headers) that get triggered on certain versions of Qt/KF5 frameworks.