perf ftrace: Add change log

Add a change log after previous enhancements.
Signed-off-by: Changbin Du <changbin.du@gmail.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt (VMware) <rostedt@goodmis.org>
Link: http://lore.kernel.org/lkml/20200808023141.14227-19-changbin.du@gmail.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-ftrace.c

@@ -3,6 +3,7 @@
  * builtin-ftrace.c
  *
  * Copyright (c) 2013  LG Electronics,  Namhyung Kim <namhyung@kernel.org>
+ * Copyright (c) 2020  Changbin Du <changbin.du@gmail.com>, significant enhancement.
  */
 
 #include "builtin.h"