xfce4: package libtool files if they are installed

This is a partial revert of commit 2ad5ceafa5 [various classes
recipes: Remove FILES entries for dbg/dev packages] to restore the
packaging of libtool (.la) files. It is generally recommended to
include 'remove-libtool' in your distro features to purge libtool
files as a do_install postfunc for all recipes, as is the documented
recommendation for handling of libtool files (unless the .la have been
determined to be absolutely necessary, in which case a recipe can use
the REMOVE_LIBTOOL_LA to prevent the cleanup). However, if the
'remove-libtool' distro feature is not specified we need to ensure
that if after a recipe completes do_install() and there are libtool
files present in the image, we must package them or we will hit
'installed-vs-shipped' QA issues.

NOTE: that even with respect to libtool files this is not a 1:1
partial revert of commit 2ad5ceafa5, adjustments are made according
to the current state of the build today.

Signed-off-by: Mark Asselstine <mark.asselstine@windriver.com>
Signed-off-by: Khem Raj <raj.khem@gmail.com>

meta-xfce/classes/xfce-panel-plugin.bbclass

@@ -11,3 +11,4 @@ FILES_${PN} += "${datadir}/xfce4/panel/plugins/"
 FILES_${PN} += "${libdir}/xfce4/panel-plugins/*.so"
 FILES_${PN} += "${libdir}/xfce4/panel/plugins/*.so"
 
+FILES_${PN}-dev += "${libdir}/xfce4/panel/plugins/*.la"