Fix deprecated this capture in omnistore/snapshot/handlers/PeriodicSyncHandler.cpp +5

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: dtolnay

Differential Revision: D81189814

fbshipit-source-id: 1c2ab90224683e129f4113a39e422f5259615482

Author: r-barnes

proxygen/lib/http/session/test/HQDownstreamSessionTest.cpp

@@ -84,7 +84,7 @@ quic::StreamId HQDownstreamSessionTest::sendHeader() {
 folly::Promise<folly::Unit> HQDownstreamSessionTest::sendRequestLater(
     proxygen::HTTPMessage req, bool eof) {
   folly::Promise<folly::Unit> reqp;
-  reqp.getSemiFuture().via(&eventBase_).thenValue([=](auto&&) {
+  reqp.getSemiFuture().via(&eventBase_).thenValue([=, this](auto&&) {
     auto id = sendRequest(req, eof);
     socketDriver_->addReadEvent(
         id, getStream(id).buf.move(), std::chrono::milliseconds(0));