Contribs: update PNG from 1.4.8 to 1.5.6

contrib/src/png/SHA512SUMS

-c7abd7c661e7ab3a59e49eb1d631dbb7ef2a538a230655fd068fbbf83ef3f21138a3593432147f6b1156971f4434f271fb97f25d2c596329c6a1a4c109d511a6  libpng-1.4.8.tar.bz2
+bcafc465c30ef1e75de4c9c5792e90e3398b8d9cb840c85bc79921531dff213ed0ed277bdcfa4feb2f378c48241566d9781302d6d6ec5fb315db8a7de3c08350  libpng-1.5.6.tar.bz2

contrib/src/png/rules.mak

 # PNG
-PNG_VERSION := 1.4.8
+PNG_VERSION := 1.5.6
 PNG_URL := $(SF)/libpng/libpng-$(PNG_VERSION).tar.bz2
 
 PKGS += png

extras/contrib/src/packages.mak

@@ -132,7 +132,7 @@ VCDIMAGER_VERSION=0.7.23
 VCDIMAGER_URL=$(GNU)/vcdimager/vcdimager-$(VCDIMAGER_VERSION).tar.gz
 CDIO_VERSION=0.80
 CDIO_URL=$(GNU)/libcdio/libcdio-$(CDIO_VERSION).tar.gz
-PNG_VERSION=1.4.8
+PNG_VERSION=1.5.6
 PNG_URL=$(SF)/libpng/libpng-$(PNG_VERSION).tar.bz2
 GPGERROR_VERSION=1.10
 #GPGERROR_URL=$(CONTRIB_VIDEOLAN)/libgpg-error-$(GPGERROR_VERSION).tar.bz2