Fix build with clang 9.0.1 and libcxx

This should not be required in C++17 because there is an appropriate
class template deduction rule [1] which infers that it's going to be a
weak_ptr<T> when constructing from a shared_ptr<T>. However, in
clang/LLVM's libcxx C++ STL implementation this only got implemented in
May 2020 [2].

[1] https://en.cppreference.com/w/cpp/memory/weak_ptr/deduction_guides
[2] https://reviews.llvm.org/D69603

src/Proxy.cpp

@@ -99,7 +99,7 @@ PendingAsyncCall Proxy::callMethod(const MethodCall& message, async_reply_handle
 
     auto callback = (void*)&Proxy::sdbus_async_reply_handler;
     auto callData = std::make_shared<AsyncCalls::CallData>(AsyncCalls::CallData{*this, std::move(asyncReplyCallback), {}});
-    auto weakData = std::weak_ptr{callData};
+    auto weakData = std::weak_ptr<AsyncCalls::CallData>{callData};
 
     callData->slot = message.send(callback, callData.get(), timeout);