layer.conf: Fix for signature change for allarch icon themes
faenza-icon-theme are allarch but recently
gtk-icon-cache, since commit 31e3d34c8a9c3e8481aa837aa0bf52adf095fe62
[gtk-icon-cache.bbclass: add runtime dependency], it start depend on
non-allarch packages gdk-pixbuf and gtk+3, which make it's signature
changes when machine changed.
Signed-off-by: Khem Raj <raj.khem@gmail.com>
Cc: Andreas Müller <schnitzeltony@gmail.com>