New optional build dependencies in Plasma 5.26

[Fusion Future]

Hello,

There are 3 additional optional build dependencies introduced in Plasma 
5.26:

1. KF5KExiv2: 
https://invent.kde.org/plasma/plasma-workspace/-/merge_requests/1852
2. KF5NetworkManagerQt: 
https://invent.kde.org/plasma/kdeplasma-addons/-/merge_requests/207
3. libicu-devel >= 66: 
https://invent.kde.org/plasma/kdeplasma-addons/-/merge_requests/112

Though the two packages can still be built without those libraries, it's 
still advised to add those build dependencies, to provide a complete 
Plasma experience.

Thank you for reading this.