Building against system KF5 Syntax Highlighting broken

Newer versions of the KF5 Syntax Highlighting library introduced a change in their include paths (when getting them via CMake), which causes the OpenTodoList build to break. This is particularly bad on Arch Linux, where by default we always build from (the latest) sources.

This is already fixed on the development branch - we shall port the fix to stable and release a bug fix version of the app ASAP.