Add detection of .mm files as Objective-C

This is a common file extension for Objective-C++ files.

src/source.cpp

@@ -83,6 +83,8 @@ Glib::RefPtr<Gsv::Language> Source::guess_language(const boost::filesystem::path
       language = language_manager->get_language("glsl");
     else if(extension == ".in" || extension == ".bak")
       return guess_language(boost::filesystem::path(file_path).replace_extension());
+    else if(extension == ".mm")
+      language = language_manager->get_language("objc");
     else if(!file_path.has_extension() && std::any_of(file_path.begin(), file_path.end(),
                                                       [](const boost::filesystem::path &path) { return path == "include"; }))
       language = language_manager->get_language("cpphdr");