[Dispatcher] replace deprecated pthread_yield with sched_yield

Compilation with gcc on ubuntu 21.10 fails because pthread_yield is
apparently deprecated and should be replaced with sched_yield.

gcc --version
gcc (Ubuntu 11.2.0-7ubuntu2) 11.2.0

emper/Dispatcher.cpp

@@ -2,7 +2,7 @@
 // Copyright © 2020 Florian Schmaus
 #include "Dispatcher.hpp"
 
-#include <pthread.h>	// for pthread_yield
+#include <sched.h>
 
 #include "Emper.hpp"
 #include "Runtime.hpp"	// for Runtime
@@ -17,6 +17,6 @@ void Dispatcher::dispatchLoopDoSleep() {
 	if constexpr (emper::WORKER_SLEEP) {
 		runtime.dispatchLoopSleep();
 	} else {
-		pthread_yield();
+		sched_yield();
 	}
 }