perf: fix buildpaths QA warning in 6.4+

kernel version 6.4 introduces a new file that need to have
absolute paths removed, so we can avoid the buildpaths QA
warning and have relocatable packages.

We add pmu-flex.h to the processing, and the issue is resolved.

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>