perf evswitch: Move struct to a separate header to use in other tools

perf evswitch: Move struct to a separate header to use in other tools

Now that we see that the simple userspace-based "slicing" of events
using delimiter events ("markers") works, lets move it to a separate
header to make it available to other tools, next step will be having
the switch on/off check done at the PERF_RECORD_SAMPLE processing
function moved too.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Florian Weimer <fweimer@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: William Cohen <wcohen@redhat.com>
Link: https://lkml.kernel.org/n/tip-z0cyi9ifzlr37cedr9xztc1k@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>