Contribs: update libpng to 1.4.8

See CVE-2011-2690 (we shouldn't be affected), CVE-2011-2691 (same),
CVE-2011-2692 (no idea)

contrib/src/png/SHA512SUMS

-f1dc2afd747554e5fda1c7627e34530d845d8c7f5629870a21f0d501604cba26446aa7a7bbeb8278f3ab714ed48cf2518df873d0e8fee4dc79d5e2b50060ce46  libpng-1.4.7.tar.bz2
+c7abd7c661e7ab3a59e49eb1d631dbb7ef2a538a230655fd068fbbf83ef3f21138a3593432147f6b1156971f4434f271fb97f25d2c596329c6a1a4c109d511a6  libpng-1.4.8.tar.bz2

contrib/src/png/rules.mak

 # PNG
-PNG_VERSION := 1.4.7
+PNG_VERSION := 1.4.8
 PNG_URL := $(SF)/libpng/libpng-$(PNG_VERSION).tar.bz2
 
 PKGS += png

extras/contrib/src/packages.mak

@@ -138,7 +138,7 @@ 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=1.4.7
+PNG_VERSION=1.4.8
 PNG_URL=$(SF)/libpng/libpng-$(PNG_VERSION).tar.bz2
 GPGERROR_VERSION=1.7
 #GPGERROR_URL=$(CONTRIB_VIDEOLAN)/libgpg-error-$(GPGERROR_VERSION).tar.bz2