[peridot] [lib] [testing] Temporarily redefine Closure in test

Before we can restore changes in:
https://fuchsia-review.googlesource.com/c/garnet/+/240405
reverted by:
https://fuchsia-review.googlesource.com/c/garnet/+/244317

We update fake_agent_runner_storage.h so it will build under
both pre-change and post-change.

Once the garnet change is restored, we will make remaining
peridot change to align with the goals to remove
fxl::MakeCopyable and replace many std::function references
with the new fit::function type.

Test: No behavior change
Change-Id: I683d1d9876f0d914cf8809eae69258e509aed653
diff --git a/lib/testing/fake_agent_runner_storage.h b/lib/testing/fake_agent_runner_storage.h
index 79b46b7..224f110 100644
--- a/lib/testing/fake_agent_runner_storage.h
+++ b/lib/testing/fake_agent_runner_storage.h
@@ -8,7 +8,6 @@
 #include <functional>
 #include <string>
 
-#include <lib/fxl/functional/closure.h>
 #include <lib/fxl/macros.h>
 
 #include "peridot/bin/sessionmgr/agent_runner/agent_runner_storage.h"
@@ -22,7 +21,7 @@
 
   // |AgentRunnerStorage|
   void Initialize(NotificationDelegate* /*delegate*/,
-                  const fxl::Closure done) override {
+                  const std::function<void()> done) override {
     done();
   }