selftests/bpf: trace_helpers.c: do not use poisoned type

selftests/bpf: trace_helpers.c: do not use poisoned type

After commit c698eaebdf47 ("selftests/bpf: trace_helpers.c: Optimize
kallsyms cache") trace_helpers.c now includes libbpf_internal.h, and
thus can no longer use the u32 type (among others) since they are poison
in libbpf_internal.h. Replace u32 with __u32 to fix the following error
when building trace_helpers.c on powerpc:

  error: attempt to use poisoned "u32"

Fixes: c698eaebdf47 ("selftests/bpf: trace_helpers.c: Optimize kallsyms cache")
Signed-off-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20240202095559.12900-1-shung-hsi.yu@suse.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>