KCMs: put the kcm_* libs in builddir/bin/kcms/
This matches the install location scheme QT_PLUGIN_DIR/kcms/, which makes it slightly easier to test stuff right from the builddir, without installation, by exporting QT_PLUGIN_DIR=builddir/bin/.
Also install kcm_fontinst in QT_PLUGIN_DIR/kcms/, like the other KCMs.