Replace platforms.linux with platforms.darwin for expressions that compile on darwin too (too restrictive platforms)

pkgs/development/python-modules/libais/default.nix

@@ -20,6 +20,6 @@ buildPythonPackage rec {
     homepage = https://github.com/schwehr/libais;
     description = "Library for decoding maritime Automatic Identification System messages";
     license = licenses.asl20;
-    platforms = platforms.linux;  # It currently fails to build on darwin
+    platforms = platforms.unix;
   };
 }