gnome-panel3: add dconf to DEPENDS

* fixes:
  | configure:14455: error: Package requirements (gdk-pixbuf-2.0 >=
  2.7.1 pango >= 1.15.4 gtk+-3.0 >= 3.0 cairo-xlib glib-2.0 >= 2.25.12
  gio-2.0 >= 2.25.12 gio-unix-2.0 >= 2.25.12 gnome-desktop-3.0 >= 2.91.0
  gconf-2.0 >= 2.6.1 libgnome-menu >= 2.27.92 dconf) were not met:
  |
  | No package 'dconf' found

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>