perf build: Conditionally add feature check flags for libtrace{event,fs}

This avoids reported warnings when the packages are not installed.

[namhyung]: Removed the dummy assignment and unnecessary ifeq checks.

Fixes: 0f0e1f44 ("perf build: Use pkg-config for feature check for libtrace{event,fs}")
Signed-off-by: Guilherme Amadio <amadio@gentoo.org>
Cc: Leo Yan <leo.yan@arm.com>
Cc: Thorsten Leemhuis <linux@leemhuis.info>
Link: https://lore.kernel.org/r/20240628203432.3273625-1-amadio@gentoo.org


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