selftests/bpf: Add missing cleanup in atomic_bounds test

Add missing skeleton destroy call.

Fixes: 37086bfdc737 ("bpf: Propagate stack bounds to registers in atomics w/ BPF_FETCH")
Reported-by: Yonghong Song <[e-mail]>
Signed-off-by: Brendan Jackman <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/prog_tests/atomic_bounds.c

···
 	skel = atomic_bounds__open_and_load();
 	if (CHECK(!skel, "skel_load", "couldn't load program\n"))
 		return;
+
+	atomic_bounds__destroy(skel);
 }