LGPLv2.1 relicensing agreements

Steven just stated clearly that he only accepts relicencing to
LGPLv2.1 (not later). So I think we might have been interpreting his
previous acceptance a little too broadly when specifying "LGPLV2.1+".
Change the licensing text to reflect this.

If some of the contributors specifically said they were OK with "or
later", we could add this back upon rechecking their original acceptance
email. The pointers to the original public emails should have been
present in the relicensing file to make our current job easier, but this
is unfortunately not the case.

Also updated the stringify.h, tracepoint.c, marker.c and trace_event.c
files.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>