libbpf: Fix single-line struct definition output in btf_dump

btf_dump APIs emit unnecessary tabs when emitting struct/union
definition that fits on the single line. Before this patch we'd get:

struct blah {<tab>};

This patch fixes this and makes sure that we get more natural:

struct blah {};

Fixes: 44a726c3

 ("bpftool: Print newline before '}' for struct with padding only fields")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20221212211505.558851-2-andrii@kernel.org