Needed to add required include directories for the feature detection
to succeed. The header tracefs.h is installed either into the include
directory /usr/include/tracefs/tracefs.h when using the Makefile, or
into /usr/include/libtracefs/tracefs.h when using meson to build
libtracefs. The header tracefs.h uses #include <event-parse.h> from
libtraceevent, so pkg-config needs to pick the correct include directory
for libtracefs and add the one for libtraceevent to succeed.

Note that in baa2ca59 the variable
LIBTRACEEVENT_DIR was introduced, and now the method to compile against
non-standard locations requires PKG_CONFIG_PATH to be set instead, which
works for both libtraceevent and libtracefs.
Signed-off-by: Guilherme Amadio <amadio@gentoo.org>
Reviewed-by: Leo Yan <leo.yan@arm.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20240606153625.2255470-2-amadio@gentoo.org