Update net-im/libqmatrixclient to v 0.5.1.1, latest upstream

Changelog is at https://github.com/QMatrixClient/libqmatrixclient/releases:

• fix linkification, fixed quirks for room names, url validation
• improved support for "upgraded" (v3) rooms

Not bumping consumers (net-im/quaternion) because there's no immediate
need, and there's a newer Quaternion release on the horizon anyway.