Calculator: Use unveil()

A barebones GUI app like this only needs read access to /res (for fonts
and themes, in case settings change) once it's up and running.

Applications/Calculator/main.cpp

@@ -47,6 +47,13 @@ int main(int argc, char** argv)
         return 1;
     }
 
+    if (unveil("/res", "r") < 0) {
+        perror("unveil");
+        return 1;
+    }
+
+    unveil(nullptr, nullptr);
+
     auto window = GWindow::construct();
     window->set_title("Calculator");
     window->set_resizable(false);