Auto-update libpng to v1.6.40 (#2222)

* Update libpng to v1.7.0beta89

* Update xmake.lua

packages/l/libpng/xmake.lua

@@ -6,6 +6,7 @@ package("libpng")
 
     set_urls("https://github.com/glennrp/libpng/archive/$(version).zip",
              "https://github.com/glennrp/libpng.git")
+    add_versions("v1.6.40", "62d25af25e636454b005c93cae51ddcd5383c40fa14aa3dae8f6576feb5692c2")
     add_versions("v1.6.37", "c2c50c13a727af73ecd3fc0167d78592cf5e0bca9611058ca414b6493339c784")
     add_versions("v1.6.36", "6274d3f761cc80f7f6e2cde6c07bed10c00bc4ddd24c4f86e25eb51affa1664d")
     add_versions("v1.6.35", "3d22d46c566b1761a0e15ea397589b3a5f36ac09b7c785382e6470156c04247f")