QBenchlib/Perf: open the perffd with PERF_FLAG_FD_CLOEXEC

This flag was introduced in the Linux kernel version 3.14. Trying to use
perf_event_open() now with a kernel older than that will cause EINVAL
errors, but 3.14 is from 2014, so most likely old enough for all of
Qt 6.

For that, I updated the copied header from v6.0.

Change-Id: I3c79b7e08fa346988dfefffd171f895201ceb4f4
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
(cherry picked from commit 8995045c62bb673af1e74bce79e3c500e0217bae)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/testlib/qbenchmarkperfevents.cpp

@@ -477,14 +477,13 @@ void QBenchmarkPerfEventsMeasurer::init()
 
 void QBenchmarkPerfEventsMeasurer::start()
 {
-
     initPerf();
     if (fd == -1) {
         // pid == 0 -> attach to the current process
         // cpu == -1 -> monitor on all CPUs
         // group_fd == -1 -> this is the group leader
         // flags == 0 -> reserved, must be zero
-        fd = perf_event_open(&attr, 0, -1, -1, 0);
+        fd = perf_event_open(&attr, 0, -1, -1, PERF_FLAG_FD_CLOEXEC);
         if (fd == -1) {
             perror("QBenchmarkPerfEventsMeasurer::start: perf_event_open");
             exit(1);