Fix deprecated this capture in folly/experimental/channels/detail/Uti…

…lity.h

Summary:
In the future LLVM will require that lambdas capture `this` explicitly. `-Wdeprecated-this-capture` checks for and enforces this now.

This diff adds an explicit `this` capture to a lambda to fix an issue that presents similarly to this:
```
   -> fbcode/path/to/my_file.cpp:66:47: error: implicit capture of 'this' with a capture default of '=' is deprecated [-Werror,-
Wdeprecated-this-capture]
   ->           detail::createIOWorkerProvider(evb, requestsRegistry_);
   ->                                               ^
   -> fbcode/path/to/my_file.cpp:61:30: note: add an explicit capture of 'this' to capture '*this' by reference
   ->   evb->runInEventBaseThread([=, self_weak = std::move(self_weak)]() {
   ->                              ^
   ->                               , this
```

Reviewed By: meyering

Differential Revision: D52279233

fbshipit-source-id: 233d60917b64d111daa13d71b213d3af7370a6ee

folly/experimental/channels/detail/Utility.h

@@ -171,7 +171,7 @@ class SenderCancellationCallback : public IChannelCallback {
    */
   void consume(ChannelBridgeBase*) override {
     cancelSource_.requestCancellation();
-    executor_->add([=]() {
+    executor_->add([=, this]() {
       CHECK(!callbackToFire_.second.isReady());
       callbackToFire_.first.setValue(CallbackToFire::Consume);
     });
@@ -183,7 +183,7 @@ class SenderCancellationCallback : public IChannelCallback {
    */
   void canceled(ChannelBridgeBase*) override {
     cancelSource_.requestCancellation();
-    executor_->add([=]() {
+    executor_->add([=, this]() {
       CHECK(!callbackToFire_.second.isReady());
       callbackToFire_.first.setValue(CallbackToFire::Canceled);
     });