CMake: find Qt5::Core at top-level

Both openSUSE and FreeBSD were failing (on april 1) in the cmake-stage with

```
CMake Error at kpartloader/CMakeLists.txt:34 (add_executable):
  Target "kpartloader" links to target "Qt5::Core" but the target was not
  found.  Perhaps a find_package() call is missing for an IMPORTED target, or
  an ALIAS target is missing?
```

with current frameworks, this is reproducible. Although I don't know
what is making Qt5::Core vanish, finding it at top-level makes
the error go away.