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.