fortuna: use a once_flag in initialize_prng()

fortuna.h

@@ -51,6 +51,8 @@ public:
 
 	auto initialize_prng() -> void {
 		try {
+			std::call_once(PRNG_init, [] { ; });
+
 			set_reseed_ctr_to_null();
 
 			std::scoped_lock sl(mtx_accu, mtx_p_pools);
@@ -116,6 +118,7 @@ public:
 
 	std::latch sync_point{1}; // wait for init before spawning the threads
 	std::latch die_point{2}; // wait for service threads to die
+	std::once_flag PRNG_init;
 
 }; // class Fortuna