xarchiver: align libexecdir so that thunar-archive-plugin finds tap files

xarchiver installs tap files for thunar-archive-plugin in
$libexexdir/thunar-archive-plugin under the assumption that there is a
single/common $libexexdir for all packages. In bitbake.conf oe-core
sets libexecdir = "${libdir}/${BPN}" which causes thunar-archive-plugin
not to find xarchiver tap-files.

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>