rtla: Define _GNU_SOURCE in timerlat_bpf.c

JIRA: https://issues.redhat.com/browse/RHEL-88653

commit 8020361d51eea5145402e450d91b083bccdcd874
Author: Tomas Glozar <tglozar@redhat.com>
Date:   Wed Apr 30 16:46:51 2025 +0200

    rtla: Define _GNU_SOURCE in timerlat_bpf.c

    Newer versions of glibc include a definition of struct sched_attr in
    bits/sched.h (included through sched.h which is included by rtla).
    Commit 0eecee340672 ("tools/rtla: fix collision with glibc
    sched_attr/sched_set_attr") has modified the definition of struct
    sched_attr in utils.h, so that it is only applied with older versions of
    glibc that do not define it, in order to prevent build failure.

    The definition in bits/sched.h depends on _GNU_SOURCE.
    timerlat_bpf.c does not define _GNU_SOURCE, making it fall back to the
    definition in utils.h. The latter has two fields less, leading to
    shifted offsets of struct timerlat_params in timerlat_bpf_init.

    Because of the shift, timerlat_bpf_init incorrectly reads
    params->entries as 0 for timerlat-hist and disables the creation of
    histogram maps, causing breakage in BPF sample collection mode:

    $ rtla timerlat hist -d 1s
    Error pulling BPF data

    Fix the issue by also defining _GNU_SOURCE in timerlat_bpf.c.

    Cc: John Kacur <jkacur@redhat.com>
    Cc: Luis Goncalves <lgoncalv@redhat.com>
    Link: https://lore.kernel.org/20250430144651.621766-1-tglozar@redhat.com
    Fixes: e34293ddcebd ("rtla/timerlat: Add BPF skeleton to collect samples")
    Signed-off-by: Tomas Glozar <tglozar@redhat.com>
    Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Signed-off-by: Tomas Glozar <tglozar@redhat.com>

tools/tracing/rtla/src/timerlat_bpf.c

@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
 #ifdef HAVE_BPF_SKEL
+#define _GNU_SOURCE
 #include "timerlat.h"
 #include "timerlat_bpf.h"
 #include "timerlat.skel.h"