Add overload to set packet free callback directly

Source/Managed/ENet.cs

@@ -281,6 +281,12 @@ namespace ENet {
 				throw new InvalidOperationException("Packet not created");
 		}
 
+		public void SetFreeCallback(IntPtr callback) {
+			CheckCreated();
+
+			Native.enet_packet_set_free_callback(nativePacket, callback);
+		}
+
 		public void SetFreeCallback(PacketFreeCallback callback) {
 			CheckCreated();
 
@@ -1021,4 +1027,4 @@ namespace ENet {
 		[DllImport(nativeLibrary, CallingConvention = CallingConvention.Cdecl)]
 		internal static extern void enet_peer_reset(IntPtr peer);
 	}
-}
+}