Register AutotoolsProject as handler for Makefiles again

95a3087a7b dropped the mime type registration by accident. Add it back to
make the plugin functional again.

Signed-off-by: Fabian Vogt <fvogt@suse.de>
Change-Id: I783429ce7883bad8b946ed1599098751d56cabad
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/plugins/autotoolsprojectmanager/autotoolsprojectplugin.cpp

@@ -91,6 +91,7 @@ class AutotoolsProjectPlugin final : public ExtensionSystem::IPlugin
 
     void initialize() final
     {
+        ProjectManager::registerProjectType<AutotoolsProject>(Constants::MAKEFILE_MIMETYPE);
         d = std::make_unique<AutotoolsProjectPluginPrivate>();
     }