gdk-pixbuf: Fix libpng determinism issues

gdk-pixbuf: Fix libpng determinism issues

We now have libpng 1.6. If we build libpng12 as well as libpng 1.6, the 1.2
version gets preferred which is not desirable and does not give deterministic builds.

We really do want to use libpng since the item in DEPENDS will provide this so
manipulate the search list so the one we DEPEND on gets chosen. This was the cause of a
recent autobuilder failure.

(From OE-Core rev: ce1d262ea36da9a9fdeeefc0ddc69833801d4d2d)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>