plugin: update desktop file install location

share/xfce4/panel-plugins has been deprecated in favor of
share/xfce4/panel/plugins.

panel-plugins/power-manager-plugin/xfce/Makefile.am

@@ -57,7 +57,7 @@ libxfce4powermanager_la_LIBADD =                     \
 #
 # Desktop file
 #
-desktopdir = $(datadir)/xfce4/panel-plugins
+desktopdir = $(datadir)/xfce4/panel/plugins
 desktop_in_in_files = power-manager-plugin.desktop.in.in
 desktop_in_files = $(desktop_in_in_files:.desktop.in.in=.desktop.in)
 desktop_DATA = $(desktop_in_files:.desktop.in=.desktop)