perf build: Warn if libtracefs is not found

Signed-off-by: Guilherme Amadio <amadio@gentoo.org>
Tested-by: Thorsten Leemhuis <linux@leemhuis.info>
Tested-by: Leo Yan <leo.yan@arm.com>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Cc: linux-trace-devel@vger.kernel.org
Link: https://lore.kernel.org/r/20240717174739.186988-2-amadio@gentoo.org


Signed-off-by: Namhyung Kim <namhyung@kernel.org>