libgpewidget: fix warning about overwritting keys

* drop FILES_${PN}-bin completely, use bindir+sbindir from bitbake.conf
* WARNING: Variable key FILES_${PN}-bin (${bindir}/* ${sbindir}/*) replaces original key FILES_libgpewidget-bin (${bindir}/*).

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>