codechecker: restore the meta.platforms attribute

This commit reinstates the `meta.platforms` attribute, which was removed
in #395969 to facilitate building on Darwin. Removing this attribute to
enable builds on Darwin was a mistake, and this commit rectifies that
error.

pkgs/by-name/co/codechecker/package.nix

@@ -123,5 +123,6 @@ python3Packages.buildPythonApplication rec {
       felixsinger
     ];
     mainProgram = "CodeChecker";
+    platforms = platforms.darwin ++ platforms.linux;
   };
 }