mqtt_client.h: add missing `<string>` include for `gcc-13`

[trofi]

Without the change build on gcc-13 fails as:

[ 98%] Built target hmain_test
In file included from /build/source/examples/mqtt/mqtt_client_test.cpp:10:
/build/source/examples/../mqtt/mqtt_client.h: In member function 'int hv::MqttClient::publish(const std::string&, const std::string&, int, int, MqttCallback)':
/build/source/examples/../mqtt/mqtt_client.h:232:25: error: invalid use of incomplete type 'const std::string' {aka 'const class std::__cxx11::basic_string<char>'}
  232 |         msg.topic_len = topic.size();
      |                         ^~~~~