Add OpenSSLKeyExchange.cpp to Cmake sources

Summary:
Context: FBOSS OSS builds depend on fizz and are currently failing with the following error message:

```
/bin/ld: /data/sandcastle/temp/skycastle/fbcode_builder_getdeps/installed/fizz/lib/libfizz.a(FizzUtil.cpp.o): in function `std::_MakeUniq<fizz::openssl::OpenSSLECKeyExchange>::__single_object std::make_unique<fizz::openssl::OpenSSLECKeyExchange, int const&, int const&>(int const&, int const&) [clone .isra.0]':
FizzUtil.cpp:(.text+0x15c): undefined reference to `vtable for fizz::openssl::OpenSSLECKeyExchange'
/bin/ld: /data/sandcastle/temp/skycastle/fbcode_builder_getdeps/installed/fizz/lib/libfizz.a(FizzUtil.cpp.o): in function `fizz::FizzUtil::createKeyExchangeFromBuf(fizz::hpke::KEMId, folly::Range<unsigned char const*>)':
FizzUtil.cpp:(.text+0xc88): undefined reference to `fizz::openssl::OpenSSLECKeyExchange::setPrivateKey(std::unique_ptr<evp_pkey_st, folly::static_function_deleter<evp_pkey_st, &EVP_PKEY_free> >)'
/bin/ld: FizzUtil.cpp:(.text+0xcea): undefined reference to `fizz::openssl::OpenSSLECKeyExchange::setPrivateKey(std::unique_ptr<evp_pkey_st, folly::static_function_deleter<evp_pkey_st, &EVP_PKEY_free> >)'
/bin/ld: FizzUtil.cpp:(.text+0xd49): undefined reference to `fizz::openssl::OpenSSLECKeyExchange::setPrivateKey(std::unique_ptr<evp_pkey_st, folly::static_function_deleter<evp_pkey_st, &EVP_PKEY_free> >)'
/bin/ld: /data/sandcastle/temp/skycastle/fbcode_builder_getdeps/installed/fizz/lib/libfizz.a(OpenSSLFactory.cpp.o): in function `std::_MakeUniq<fizz::openssl::OpenSSLECKeyExchange>::__single_object std::make_unique<fizz::openssl::OpenSSLECKeyExchange, int const&, int const&>(int const&, int const&) [clone .isra.0]':
OpenSSLFactory.cpp:(.text+0x1b7): undefined reference to `vtable for fizz::openssl::OpenSSLECKeyExchange'
/bin/ld: /data/sandcastle/temp/skycastle/fbcode_builder_getdeps/installed/fizz/lib/libfizz.a(Utils.cpp.o): in function `std::_MakeUniq<fizz::openssl::OpenSSLECKeyExchange>::__single_object std::make_unique<fizz::openssl::OpenSSLECKeyExchange, int const&, int const&>(int const&, int const&) [clone .isra.0]':
Utils.cpp:(.text+0x18): undefined reference to `vtable for fizz::openssl::OpenSSLECKeyExchange'
collect2: error: ld returned 1 exit status
[1064/1080] Building CXX object CMakeFiles/fsdb_handler.dir/fboss/fsdb/server/ServiceHandler.cpp.o
FAILED: CMakeFiles/fsdb_handler.dir/fboss/fsdb/server/ServiceHandler.cpp.o
```

taken from: https://www.internalfb.com/sandcastle/job/18014399822757787?no_redirect=true

This seems to be caused by the fact that OpenSSLKeyExchange.cpp was recently added, but was not added to the OSS build via CMakeLists.txt, so we add it in this PR.

Reviewed By: zalecodez

Differential Revision: D58099797

fbshipit-source-id: 2dcb05192f796a087786c34f3280f8996157f612

fizz/CMakeLists.txt

@@ -192,6 +192,7 @@ set(FIZZ_SOURCES
   crypto/hpke/Hpke.cpp
   crypto/hpke/Utils.cpp
   backend/openssl/crypto/OpenSSLKeyUtils.cpp
+  backend/openssl/crypto/exchange/OpenSSLKeyExchange.cpp
   record/Types.cpp
   record/RecordLayer.cpp
   record/EncryptedRecordLayer.cpp