pngcheck: fix meta.platforms not being flattened, remove zlib overwrite

zlib is build with static by default

pkgs/tools/graphics/pngcheck/default.nix

···
     homepage = "http://pmt.sourceforge.net/pngcrush";
     description = "Verifies the integrity of PNG, JNG and MNG files";
     license = licenses.free;
-    platforms = with platforms; [ unix ];
+    platforms = platforms.unix;
     maintainers = with maintainers; [ starcraft66 ];
   };
 }

pkgs/top-level/all-packages.nix

···
 
   pm2 = nodePackages.pm2;
 
-  pngcheck = callPackage ../tools/graphics/pngcheck {
-    zlib = zlib.override {
-      static = true;
-    };
-  };
+  pngcheck = callPackage ../tools/graphics/pngcheck { };
 
   pngcrush = callPackage ../tools/graphics/pngcrush { };