perf build: Add missing comment about NO_LIBTRACEEVENT=1

perf build: Add missing comment about NO_LIBTRACEEVENT=1

By default perf will fail the build if the development files for
libtraceevent are not available.

To build perf without libtraceevent support, disabling several features
such as 'perf trace', one needs to add NO_LIBTRACEVENT=1 to the make
command line.

Add the missing comments about that to the tools/perf/Makefile.perf
file, just like all the other such command line toggles.

Fixes: 378ef0f5d9d7f465 ("perf build: Use libtraceevent from the system")
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Reviewed-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/ZR6+MhXtLnv6ow6E@kernel.org
Signed-off-by: Namhyung Kim <namhyung@kernel.org>