conversation: fix build error on Windows

It seems that msvc doesn't support using the '+' operator to concatenate
an std::filesystem::path and a string, but that the '+=' operator works.

GitLab: #1118
Change-Id: I153b47458c5ec8c690cea9cd173c9054b115090d

src/jamidht/conversation.cpp

@@ -2177,7 +2177,8 @@ Conversation::downloadFile(const std::string& interactionId,
                 }
             }
 
-            auto tempFilePath = filePath + std::string(".tmp");
+            std::filesystem::path tempFilePath(filePath);
+            tempFilePath += ".tmp";
             auto start = fileutils::size(tempFilePath);
             if (start < 0)
                 start = 0;