rtla: Define _GNU_SOURCE in timerlat_bpf.c

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>