dual license ftrace.txt

I asked legal about the licensing of ftrace.txt, and they told me that,
unless the Documentation directory is specifically set up to handle non
GPL licenses (which it does not appear to be), then it would be best to
put ftrace.txt under the GPL.

This patch adds a dual license to ftrace.txt such that it is under both
the FDL and the GPL.
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/ftrace.txt

@@ -4,6 +4,7 @@
 Copyright 2008 Red Hat Inc.
    Author:   Steven Rostedt <srostedt@redhat.com>
   License:   The GNU Free Documentation License, Version 1.2
+               (dual licensed under the GPL v2)
 Reviewers:   Elias Oltmanns, Randy Dunlap, Andrew Morton,
 	     John Kacur, and David Teigland.