Fix Windows build

Author: bugdea1er

tests/directory.cpp

@@ -23,7 +23,7 @@ TEST(directory, create_with_prefix) {
   EXPECT_TRUE(fs::is_directory(tmpdir));
   EXPECT_TRUE(fs::equivalent(parent, fs::temp_directory_path()));
 
-  std::string filename = tmpdir.path().filename();
+  fs::path::string_type filename = tmpdir.path().filename();
   EXPECT_EQ(filename.substr(0, std::string(PREFIX).length()), PREFIX);
 
   fs::perms permissions = fs::status(tmpdir).permissions();