fix extra compiler warnings

this is only relevant people who are masochistic enough to go beyond
`-Wall`. (note that at this warning level, there is plenty of noise from
Dune and other upstream dependencies.)

tests/models/test_tasklets.cpp

@@ -62,6 +62,7 @@ private:
     int mseconds_;
 };
 
+void sleepAndPrintFunction();
 void sleepAndPrintFunction()
 {
     int ms = 100;