client: translations: improve Makefile * Change target `translations` to only be remade if one of the `*.po` files changes. Technically is only directly depends on these files. This looks like a defect in the original upstream Makefile logic. * Add found `*.js` files to the dependencies of the `pot` target, because they are used when creating it. This looks like it was missed when adding the Makefile initially with 0fd572582ab8.