Old thread, but since this thread shows up high in a Google search and since .resizable(resizingMode: .tile) won't work with system symbols, starting with iOS 15 we can do the following:

struct ContentView: View {
    var body: some View {
        Rectangle()
            .foregroundStyle(.image(
                Image(systemName: "questionmark.circle")
            ))
            .font(.system(size: 50))
    }
}

Full credit to this blog post I've found: https://fatbobman.com/en/posts/how-to-tile-images-in-swiftui/