XdgIconLoader: Fix FTBFS in super-build/in-tree builds
XdgIconLoader: Fix FTBFS in super-build/in-tree builds

We were not copying the XdgIconLoader private installable headers.

lxde/libqtxdg,luis-pereira/libqtxdg,lxde/libqtxdg,libqtxdg/libqtxdg,luis-pereira/libqtxdg,libqtxdg/libqtxdg