add 'hf plot' - from offical repo (piwi)

client/cmdhf.h

@@ -1,6 +1,6 @@
 //-----------------------------------------------------------------------------
 // Copyright (C) 2010 iZsh <izsh at fail0verflow.com>
-//
+// Piwi, Feb 2019
 // This code is licensed to you under the terms of the GNU GPL, version 2 or,
 // at your option, any later version. See the LICENSE.txt file for the text of
 // the license.
@@ -17,5 +17,6 @@ int CmdHF(const char *Cmd);
 int CmdHFTune(const char *Cmd);
 int CmdHFSearch(const char *Cmd);
 int CmdHFSniff(const char *Cmd);
+int CmdHFPlot(const char *Cmd);
 
 #endif