perf build: Add missing cflags when building with custom libtraceevent

When building with custom libtraceevent, below errors occur:

$ make -C tools/perf NO_LIBPYTHON=1 PKG_CONFIG_PATH=<custom libtraceevent>
In file included from util/session.h:5,
from builtin-buildid-list.c:17:
util/trace-event.h:153:10: fatal error: traceevent/event-parse.h: No such file or directory
153 | #include <traceevent/event-parse.h>
| ^~~~~~~~~~~~~~~~~~~~~~~~~~
<snip similar errors of missing headers>

This is because the include path is missed in the cflags. Add it.

Fixes: 0f0e1f445690 ("perf build: Use pkg-config for feature check for libtrace{event,fs}")
Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
Reviewed-by: Leo Yan <leo.yan@arm.com>
Reviewed-by: Guilherme Amadio <amadio@gentoo.org>
Cc: linuxarm@huawei.com
Link: https://lore.kernel.org/r/20241024133236.31016-1-yangyicong@huawei.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>