Implement IEquatable<T> for AsyncFlowControl

Similar to https://github.com/dotnet/runtime/issues/2127, all the required members for implementing IEquatable already exist, it just needs to be marked as so in the source. 
```diff
namespace System.Threading
{
-   public struct AsyncFlowControl : IDisposable
+   public struct AsyncFlowControl : IEquatable<AsyncFlowControl>, IDisposable
    {
```