Adds missing ${CMAKE_INSTALL_PREFIX}
Adds missing ${CMAKE_INSTALL_PREFIX}

Introduced by commit 5a3027c93eb5f234537d0d58cc87df2fa80f5f3a

Signed-off-by: Luís Pereira <95d1463b1ff9eef535e39fb0d755bc242cfcea61@gmail.com>

gilir/libqtxdg-qt5,luis-pereira/libqtxdg,libqtxdg/libqtxdg,DrTobe/libqtxdg,DrTobe/libqtxdg,DrTobe/libqtxdg,lxde/libqtxdg,libqtxdg/libqtxdg,gilir/libqtxdg-qt5,lxde/libqtxdg,luis-pereira/libqtxdg