Ensure creation of shutdownGuard on main thread

Minimizes errors and allows us to use shutdownGuard as "an object on the
main thread" from other threads.

Change-Id: I8bca128a3fe7d6ed9ca585891557af729c5479bb
Reviewed-by: hjk <hjk@qt.io>

src/libs/extensionsystem/pluginmanager.cpp

@@ -274,6 +274,7 @@ PluginManager::PluginManager()
 {
     m_instance = this;
     d = new PluginManagerPrivate(this);
+    shutdownGuard(); // ensure creation on main thread
 }
 
 /*!