Define Event type completely

The Event type must be known because we call it from the inline template
method. The alternative is worse, we'd have to instantiate the template
for every possible simulator type in the .cpp!

opm/core/simulator/SimulatorOutput.hpp

@@ -20,6 +20,9 @@
 #ifndef OPM_SIMULATOR_OUTPUT_HPP
 #define OPM_SIMULATOR_OUTPUT_HPP
 
+// need complete def. of this since we use it in template
+#include <opm/core/utility/Event.hpp>
+
 #include <memory>  // unique_ptr, shared_ptr
 #include <vector>