libpng: update SRC_URI back to SF

libpng: update SRC_URI back to SF

ERROR: Task 944 (virtual:nativesdk:/home/akuster/oss/maint/poky/meta/recipes-multimedia/libpng/libpng_1.6.21.bb, do_checkuri) failed with exit code '1'
ERROR: libpng12-1.2.56-r0 do_checkuri: Function failed: Fetcher failure for URL: 'http://distfiles.gentoo.org/distfiles/libpng-1.2.56.tar.xz'. URL http://distfiles.gentoo.org/distfiles/libpng-1.2.56.tar.xz doesn't work
ERROR: Logfile of failure stored in: /home/akuster/oss/maint/poky/build/tmp/work/i586-poky-linux/libpng12/1.2.56-r0/temp/log.do_checkuri.14781
Log data follows:
| DEBUG: Executing python function do_checkuri
| DEBUG: Testing URL http://distfiles.gentoo.org/distfiles/libpng-1.2.56.tar.xz
| DEBUG: checkstatus() urlopen failed: HTTP Error 404: Not Found
| DEBUG: Python function do_checkuri finished
| ERROR: Function failed: Fetcher failure for URL: 'http://distfiles.gentoo.org/distfiles/libpng-1.2.56.tar.xz'. URL http://distfiles.gentoo.org/distfiles/libpng-1.2.56.tar.xz doesn't work

SF now has a old releases dir which contains this tarball. It got dropped from Gentoo

Signed-off-by: Armin Kuster <akuster808@gmail.com>