[favoriteproxytest] Link against KConfig

This happens to work because of transitive linkage, but we better not
rely on that.
1 job for master in 13 minutes and 1 second (queued for 115 minutes and 9 seconds)
Status Name Job ID Coverage
  Build
passed linux kf5-qt5 qt5.15 #99153

00:13:01

1.0%