contribs: png: disable debug lib suffix for all platforms

Given contribs are bootstrapped with --disable-optim option, the CMAKE flag -DCMAKE_BUILD_TYPE is set to Debug.

When build configuration is set to Debug in libpng contrib, it CMAKE configuration append a d char to the static library name, resulting in a lib named libpng16d.a instead of libpng16.a.

Then some vlc module depending on libpng aren't able to link properly as they can't find their -lpng16 dependency.