bpftool: Print newline before '}' for struct with padding only fields

btf_dump_emit_struct_def attempts to print empty structures at a
single line, e.g. `struct empty {}`. However, it has to account for a
case when there are no regular but some padding fields in the struct.
In such case `vlen` would be zero, but size would be non-zero.

E.g. here is struct bpf_timer from vmlinux.h before this patch:

 struct bpf_timer {
 	long: 64;
	long: 64;};

And after this patch:

 struct bpf_dynptr {
 	long: 64;
	long: 64;
 };
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221001104425.415768-1-eddyz87@gmail.com

tools/lib/bpf/btf_dump.c

@@ -944,7 +944,11 @@ static void btf_dump_emit_struct_def(struct btf_dump *d,
 					  lvl + 1);
 	}
 
-	if (vlen)
+	/*
+	 * Keep `struct empty {}` on a single line,
+	 * only print newline when there are regular or padding fields.
+	 */
+	if (vlen || t->size)
 		btf_dump_printf(d, "\n");
 	btf_dump_printf(d, "%s}", pfx(lvl));
 	if (packed)