Fix fs::path.c_str() being wchar on Windows

test/test_clinkahead.cpp

@@ -591,7 +591,7 @@ TEST_F(test_clinkahead, test_up_n_download_file) {
   linkahead_entity_entity original_entity;
   linkahead_entity_create_entity(&original_entity);
   linkahead_entity_entity_set_role(&original_entity, "FILE");
-  linkahead_entity_entity_set_local_path(&original_entity, test_upload_file_1.c_str());
+  linkahead_entity_entity_set_local_path(&original_entity, test_upload_file_1.string().c_str());
   linkahead_entity_entity_set_file_path(&original_entity, "/some/file");
 
   linkahead_transaction_transaction insert_transaction;
@@ -619,7 +619,7 @@ TEST_F(test_clinkahead, test_up_n_download_file) {
   // return_code = linkahead_transaction_transaction_retrieve_by_id(
   //   &retrieve_transaction_1, rec_id);
   return_code = linkahead_transaction_transaction_retrieve_and_download_file_by_id(
-    &retrieve_transaction_1, rec_id, test_download_file_1.c_str());
+    &retrieve_transaction_1, rec_id, test_download_file_1.string().c_str());
   EXPECT_EQ(return_code, linkahead::StatusCode::GO_ON);
   return_code = linkahead_transaction_transaction_execute(&retrieve_transaction_1);
   EXPECT_EQ(return_code, 0);