Update libpng reference to 1.6.17
Update libpng reference to 1.6.17

Change-Id: Idef27a35ca598c1df95f1c38392aa0450322392a

KhronosGroup/Vulkan-CTS,chadversary/deqp,geekboxzone/mmallow_external_deqp,geekboxzone/mmallow_external_deqp,chadversary/deqp,googlestadia/VK-GL-CTS,KhronosGroup/VK-GL-CTS,googlestadia/VK-GL-CTS,KhronosGroup/Vulkan-CTS,chadversary/deqp,KhronosGroup/VK-GL-CTS,geekboxzone/mmallow_external_deqp,geekboxzone/mmallow_external_deqp,KhronosGroup/VK-GL-CTS,googlestadia/VK-GL-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/Vulkan-CTS,googlestadia/VK-GL-CTS,googlestadia/VK-GL-CTS,geekboxzone/mmallow_external_deqp,KhronosGroup/VK-GL-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/Vulkan-CTS,geekboxzone/mmallow_external_deqp,chadversary/deqp,chadversary/deqp,googlestadia/VK-GL-CTS,chadversary/deqp,KhronosGroup/Vulkan-CTS