new libpng packages

[Riccardo Mottola via buildfarm]

Hi,

libpng_dev
libpng16_16

please install the two packages above on unstable9s, unstable9x, 
unstable10s, unstable10x

and let's cross fingers.

I think the util packages are only for the end-user and not useful on a 
build farm.

Riccardo