Fix include: Add <ostream> include to "Layer.h". (#2100)

std::ostream is used in the overload for operator<<(ostream, Layer). That will fail compilation if ostream is not defined.

Author: Dimi1010

Packet++/header/Layer.h

@@ -3,6 +3,7 @@
 #include <stdint.h>
 #include <stdio.h>
 #include "ProtocolType.h"
+#include <ostream>
 #include <string>
 #include <stdexcept>
 #include <utility>