Add missing include for sstream, required for std::ostringstream

[Tobias-Fischer]

Related build failure:

/home/tobias/mambaforge/conda-bld/carlsim6_1711409603953/work/carlsim/kernel/src/snn_cpu_module.cpp: In member function 'void SNN::findWavefrontPath_CPU(std::vector<int>&, std::vector<float>&, int, int, int, int)':
/home/tobias/mambaforge/conda-bld/carlsim6_1711409603953/work/carlsim/kernel/src/snn_cpu_module.cpp:1025:36: error: aggregate 'std::ostringstream string_stream' has incomplete type and cannot be defined
 1025 |                 std::ostringstream string_stream;
      |                                    ^~~~~~~~~~~~~

[larsnm]

Hello Tobias, Thank you very much for your contribution to CARLsim.
We are reviewing your pull request and let you know about any updates.
Kind regards,
Your CARLsim Team