MINIFICPP-2506 Fix a gcc warning

[fgerlits]

There is a GCC warning in PythonLibLoader.cpp, so minifi does not compile with ENABLE_PYTHON_SCRIPTING and MINIFI_FAIL_ON_WARNINGS both ON.

https://issues.apache.org/jira/browse/MINIFICPP-2506

The warning:

/home/runner/work/nifi-minifi-cpp/nifi-minifi-cpp/extensions/python/pythonlibloader/PythonLibLoader.cpp: In static member function ‘static std::string PythonLibLoader::execCommand(const std::string&)’:
/home/runner/work/nifi-minifi-cpp/nifi-minifi-cpp/extensions/python/pythonlibloader/PythonLibLoader.cpp:78:44: error: ignoring attributes on template argument ‘int (*)(FILE*)’ [-Werror=ignored-attributes]
   78 |     std::unique_ptr<FILE, decltype(&pclose)> pipe(popen(cmd.c_str(), "r"), pclose);
      |                                            ^

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