qemu: Ensure libpng dependency is deterministic

If the host has libpng, because of the way we handle the GL dependency in qemu,
it can cause determinism issues. Add a specific PACKAGECONFIG entry for libpng
to avoid this (and the associated autobuilder/uninative glibc symbol mismatch
failures).

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 34afdd0b)
Signed-off-by: Steve Sakoman <steve@sakoman.com>