Enable default construction of PendingAsyncCall (#180)

This is helpful in use cases where a user defined class wants to
store a PendingAsyncCall as a member variable, or in a STL
container.

include/sdbus-c++/IProxy.h

@@ -293,6 +293,8 @@ namespace sdbus {
     class PendingAsyncCall
     {
     public:
+        PendingAsyncCall() = default;
+
         /*!
          * @brief Cancels the delivery of the pending asynchronous call result
          *