Fix FTBS of nm-tray

Summary:

• Add missing include for used class
• Remove unnecessary dependency on libkf5networkmanagerqt-dev

Test Plan:

1. Check that the build does not fail.
   1. Check that it has the same behaviour as before.

Reviewers: wxl, tsimonq2, hmollercl

Differential Revision: https://phab.lubuntu.me/D48