Add conditional Foundation import to swift.jinja2 template

Xcode started complaining about this while trying to consume Glean via the AppServices megazord (which is required for A-S components to use the Glean Rust bindings on iOS). I purposefully made this conditional to protect against the cases where Foundation may not be importable/available, which I _think_ should keep this working in our sample app which doesn't consume Glean from rust-components-swift.

Author: travis79

glean_parser/templates/swift.jinja2

@@ -4,6 +4,10 @@
 {# The rendered markdown is autogenerated, but this
 Jinja2 template is not. Please file bugs! #}
 
+#if canImport(Foundation)
+    import Foundation
+#endif
+
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */