kernel/server_session: Return a std::pair from CreateSessionPair()

Keeps the return type consistent with the function name. While we're at
it, we can also reduce the amount of boilerplate involved with handling
these by using structured bindings.

src/core/hle/kernel/server_session.cpp

@@ -204,6 +204,6 @@ ServerSession::SessionPair ServerSession::CreateSessionPair(KernelCore& kernel,
     client_session->parent = parent;
     server_session->parent = parent;
 
-    return std::make_tuple(std::move(server_session), std::move(client_session));
+    return std::make_pair(std::move(server_session), std::move(client_session));
 }
 } // namespace Kernel