[bugfix] add missing include visibility.hh

When using DUNE_EXPORT, you need to include visibility.hh. This was
omitted when adding this flag to the Singleton class in singleton.hh.
This patch adds the missing include.

dune/common/singleton.hh

@@ -5,6 +5,8 @@
 
 #include <memory>
 
+#include <dune/common/visibility.hh>
+
 /**
  * @file
  * @brief Useful wrapper for creating singletons.