fix net-im/qtox build - missing dependency

[This recent commit](https://github.com/tux3/qTox/commit/e01467e386a7b46a83ff7c7de0bbf6b7e3a089e0) in the qtox codebase added a dependency to qtxml-5 to the project. This PR contains the changes required to the ebuild to function properly. Similar to the other QT dependencies, qtxml-5 is available from the 'qt' overlay.