libpng12: fix clang-19 build (#362685)

pkgs/development/libraries/libpng/12.nix

···
     sha256 = "1izw9ybm27llk8531w6h4jp4rk2rxy2s9vil16nwik5dp0amyqxl";
   };
 
+  postPatch = lib.optionalString stdenv.hostPlatform.isDarwin ''
+    substituteInPlace pngconf.h --replace-fail '<fp.h>' '<math.h>'
+  '';
+
   outputs = [ "out" "dev" "man" ];
 
   propagatedBuildInputs = [ zlib ];