Fixed call to directory_entry.is_regular_file to work on older Boost versions

The method boost::filesytem::directory_entry.is_regular_file was added
starting in version 1.83, but the lowest Boost version of this project
is 1.66.0.

Author: jmickelin

src/slic3r/GUI/BackgroundSlicingProcess.cpp

@@ -120,7 +120,7 @@ BackgroundSlicingProcess::~BackgroundSlicingProcess()
 	std::string prefix = boost::filesystem::path(m_temp_output_path).filename().string();
 	prefix = prefix.substr(0, prefix.find('_'));
     for (const auto& entry : boost::filesystem::directory_iterator(temp_dir)) {
-        if (entry.is_regular_file()) {
+        if (boost::filesystem::is_regular_file(entry.path())) {
             const std::string filename = entry.path().filename().string();
             if (boost::starts_with(filename, prefix) && boost::ends_with(filename, ".gcode"))
                 boost::filesystem::remove(entry);