QDBusReply<void>: Fix warning about implicit copy constructor

Provide a copy constructor, fixing:

src/corelib/kernel/qmetatype.h:2702:11: warning: implicitly-declared QDBusReply<void>::QDBusReply(const QDBusReply<void>&) is deprecated [-Wdeprecated-copy]

Pick-to: 5.15
Change-Id: I725be78ace9d1b31fa2de9a23a82f92d5969dc27
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/dbus/qdbusreply.h

@@ -172,6 +172,8 @@ public:
         return *this;
     }
 
+    inline QDBusReply(const QDBusReply &) = default;
+
     inline QDBusReply& operator=(const QDBusReply& other)
     {
         m_error = other.m_error;