Convert Device::serial_number() to an attribute string as for other
functions such as vendor() and model().
Signed-off-by: Marc Schink <redacted>
%attributestring(sigrok::Device, std::string, vendor, vendor);
%attributestring(sigrok::Device, std::string, model, model);
%attributestring(sigrok::Device, std::string, version, version);
+%attributestring(sigrok::Device, std::string, serial_number, serial_number);
%attributevector(Device,
std::vector<std::shared_ptr<sigrok::Channel> >,