Fix kcmfontinst install destination

This reverts part of a8ec9541, KCModuleLoader can't load kcm_fontinst
if the library is installed in KDE_INSTALL_PLUGINDIR/kcms/; looks like that
only works for KCMs using QML.

BUG: 436306
FIXED-IN: 5.22.1
(cherry picked from commit 81ae7f8e)