Fix deprecated this capture in c2p/secure_thrift/cpp/proxy/http/tests/WebSocketProxyServerTest.cpp

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
```

Differential Revision: D80966357

fbshipit-source-id: f4dcd26da4d53094a315e3506d799191d1d3c04c

Author: r-barnes

cachelib/navy/serialization/RecordIO.cpp

@@ -110,7 +110,7 @@ class DeviceMetaDataWriter final : public RecordWriter {
     auto dataOffset = 0;
     uint8_t* bufferData = buffer_.data();
 
-    auto flushBuffer = [=]() mutable {
+    auto flushBuffer = [=, this]() mutable {
       auto extraBytes = blockSize_ - bufIndex_;
       // zero the unused bytes in the buffer
       memset(&bufferData[bufIndex_], 0, extraBytes);