Add missing import (flutter/engine#47083)

Fixes the following:

c8aa0844f2 results in the following error appearing internally when compiling for Android:

```
In file included from fml/synchronization/sync_switch.cc:5:
./fml/synchronization/sync_switch.h:74:34: error: implicit instantiation of undefined template 'std::vector<fml::SyncSwitch::Observer *>'
  mutable std::vector<Observer*> observers_;
                                 ^
<...>: note: template is declared here
class _LIBCPP_TEMPLATE_VIS vector;
                           ^
1 error generated.
```

*List which issues are fixed by this PR. You must list at least one issue.*
b/306266542

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

engine/src/flutter/fml/synchronization/sync_switch.h

@@ -5,9 +5,9 @@
 #ifndef FLUTTER_FML_SYNCHRONIZATION_SYNC_SWITCH_H_
 #define FLUTTER_FML_SYNCHRONIZATION_SYNC_SWITCH_H_
 
-#include <forward_list>
 #include <functional>
 #include <memory>
+#include <vector>
 
 #include "flutter/fml/macros.h"
 #include "flutter/fml/synchronization/shared_mutex.h"