doesn't build with libpng15 or libpng16

[nieder]

src/rwpng.c doesn't build when using libpng15 or libpng16 because the macro ZLIB_VERSION is not found. This is because png.h in these newer versions no longer #include <zlib.h> themselves. Adding "#include <zlib.h>" to src/rwpng.c directly allows the build to finish and pngnq to work.