build_sys: mpng_test depends on zlib

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

configure

@@ -1790,6 +1790,7 @@ test_deps _muxer _demuxer                                               \
 ac3_fixed_test_deps="ac3_fixed_encoder ac3_decoder"
 flashsv2_test_deps="zlib"
 mpg_test_deps="mpeg1system_muxer mpegps_demuxer"
+mpng_test_deps="zlib"
 pp_test_deps="mp_filter"
 pp2_test_deps="mp_filter"
 pp3_test_deps="mp_filter"