libpng: whitelist CVE-2019-17371

This is actually a memory leak in gif2png 2.x, so whitelist it in the libpng
recipe.

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>