Normalize plugin filename so output doesn't look wonky on Windows.

Author: clangen

src/musikcore/plugin/PluginFactory.cpp

@@ -101,7 +101,7 @@ void PluginFactory::LoadPlugins() {
         fs::directory_iterator end;
         for (fs::directory_iterator file(dir); file != end; file++) {
             if (fs::is_regular_file(file->status())){
-                std::string filename(file->path().u8string());
+                std::string filename(fs::path(file->path()).make_preferred().u8string());
 
                 std::shared_ptr<Descriptor> descriptor(new Descriptor());
                 descriptor->filename = filename;