gdk-pixbuf: copy loaders.cache later on

gdk-pixbuf: copy loaders.cache later on

Trying to copy loaders.cache from host-gdk-pixbuf to the gdk-pixbuf
build directory in the post-patch hook is too early when using TLP (it
breaks horribly) since host-gdk-pixbuf isn't built yet during the
massive unpack/patch cycle.
Switch it to the pre-build hook instead which ensures that gdk-pixbuf
dependencies were already built.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>