chore: fix warning

Author: muffgaga

nestkernel/connection.h

@@ -45,6 +45,8 @@
 #include "dictutils.h"
 #include "doubledatum.h"
 
+#include <cstdlib>
+
 namespace nest
 {
 
@@ -134,7 +136,7 @@ class Connection
    * To prevent erronous calls of this function on primary connections, the base class implementation
    * below just contains `assert(false)`.
    */
-  SecondaryEvent* get_secondary_event();
+  [[noreturn]] SecondaryEvent* get_secondary_event();
 
   /**
    * Get all properties of this connection and put them into a dictionary.
@@ -401,6 +403,7 @@ SecondaryEvent*
 Connection< targetidentifierT >::get_secondary_event()
 {
   assert( false );
+  std::abort();
 }
 
 } // namespace nest