selftests/bpf: Add missing cleanup in atomic_bounds test

Add missing skeleton destroy call.

Fixes: 37086bf ("bpf: Propagate stack bounds to registers in atomics w/ BPF_FETCH")
Reported-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Brendan Jackman <jackmanb@google.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210208123737.963172-1-jackmanb@google.com

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

@@ -12,4 +12,6 @@ void test_atomic_bounds(void)
 	skel = atomic_bounds__open_and_load();
 	if (CHECK(!skel, "skel_load", "couldn't load program\n"))
 		return;
+
+	atomic_bounds__destroy(skel);
 }