libpng12: update URL that no longer exists
Fix the following warning:
WARNING: Failed to fetch URL http://downloads.sourceforge.net/project/
libpng/libpng12/1.2.53/libpng-1.2.53.tar.xz, attempting MIRRORS if
available.
[YOCTO #8739]
Signed-off-by: Maxin B. John <maxin.john@intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>