Fix unreferenced local variable

Summary:
Some warnings popped up in my local Windows build today:
```
C:\open\x\xplat\wangle\acceptor\Acceptor.cpp(379): warning C4101: 'ex': unreferenced local variable
C:\open\x\xplat\wangle\acceptor\Acceptor.cpp(384): warning C4101: 'ex': unreferenced local variable
```

Differential Revision: D50139332

fbshipit-source-id: 235c8e9e2691b18ed1c748209d82d234d4e88483

wangle/acceptor/Acceptor.cpp

@@ -376,12 +376,12 @@ static std::string logContext(folly::AsyncTransport& transport) {
   int socketFd = -1;
   try {
     localAddr = transport.getLocalAddress().describe();
-  } catch (folly::AsyncSocketException& ex) {
+  } catch (folly::AsyncSocketException&) {
     localAddr = "(unknown)";
   }
   try {
     remoteAddr = transport.getPeerAddress().describe();
-  } catch (folly::AsyncSocketException& ex) {
+  } catch (folly::AsyncSocketException&) {
     remoteAddr = "(unknown)";
   }