x86, ftrace: call trace->open() before stopping tracing; add trace->print_header()

Add a callback to allow an ftrace plug-in to write its own header.

Move the call to trace->open() up a few lines.

The changes are required by the BTS ftrace plug-in.

Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>