Fix deprecated this capture in folly/experimental/channels/ProxyChann…

…el-inl.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: D52279203

fbshipit-source-id: b897d027b0c238247e0686286325eb555dce8963

folly/experimental/channels/ProxyChannel-inl.h

@@ -155,7 +155,7 @@ class ProxyChannelProcessor : public IChannelCallback {
    * a value from an input receiver or a cancellation from the output receiver).
    */
   void consume(ChannelBridgeBase* bridge) override {
-    executor_->add([=]() {
+    executor_->add([=, this]() {
       auto state = state_.wlock();
       if (bridge == state->sender.get()) {
         // The consumer of the output receiver has stopped consuming.