Fix: don't do macro expansion in tracepoint file name

Fix: don't do macro expansion in tracepoint file name

Change the API in a backward compatible way: previously, probe headers
would do e.g.

but e.g. "linux" here would be expanded to "1", which is bad.

Fix this by allowing those headers to do:

instead. We still support the old TRACEPOINT_INCLUDE_FILE for backward
compatibility.

Reported-by: Sébastien Barthélémy <barthelemy@crans.org>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>