Try to workaround LLD for Mach-O exports trie generation error

Summary:
There seems to be some kind of exports trie generation error under some
circumstances for the library `header_client_channel`. LLD seems to generate
a malformed exports trie for it, as one can verify using `llvm-objdump`.

```
llvm-objdump: error: 'buck-out/v2/gen/fbcode/4dabced598179ca5/thrift/lib/cpp2/async/__header_client_channel__/libheader_client_channel-2ed4355.dylib': truncated or malformed object (loop in childern in export trie data at node: 0x5f back to node: 0x13b2)
```

Adding this extra variable should not really affect the export trie (it is not
used anywhere, so it is not exported), but it seems to avoid the input values
that seems to create an invalid trie.

Reviewed By: narissiam

Differential Revision: D79464659

fbshipit-source-id: 6fe43ccfc8a8436ca8d949e2124da48cbc8c7832

Author: drodriguez

third-party/thrift/src/thrift/lib/cpp2/type/Protocol.h

@@ -111,6 +111,10 @@ class Protocol : public detail::Wrap<ProtocolUnion, union_t<ProtocolUnion>> {
 // Raises std::invalid_argument on failure.
 void validateProtocol(const Protocol& protocol);
 
+// FIXME(T232815335): Need a dummy (unused) variable to fix the exports trie
+// generation bug of LLD for Mach-O
+inline const void* kDummy = &kDummy;
+
 FOLLY_PUSH_WARNING
 FOLLY_CLANG_DISABLE_WARNING("-Wglobal-constructors")
 inline const Protocol kNoProtocol = {};