bpftool: Don't drop a type in the sorted C dump

The C dump sorts types by default, so that generated headers are
diffable. The sorted dump emits one type fewer than the unsorted dump
of the same BTF.

dump_btf_c() starts its loop at index 1 to skip the void type at BTF
type ID 0. That holds for the unsorted dump, where the array index is
the type ID, but not after qsort(): position 0 is then the lowest
ranked type, and btf_type_rank() ranks an anonymous enum 0 while void
takes the default rank of 10. So the enum is skipped, and void is
emitted instead as a no-op.

Fixes: 94133cf24b ("bpftool: Introduce btf c dump sorting")
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://lore.kernel.org/r/20260828215207.3105313-7-ihor.solodrai@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/bpf/bpftool/btf.c

@@ -805,9 +805,13 @@ static int dump_btf_c(const struct btf *btf,
 
 		if (sort_dump)
 			datums = sort_btf_c(btf);
-		for (i = 1; i < cnt; i++) {
+		for (i = 0; i < cnt; i++) {
 			int idx = datums ? datums[i].index : i;
 
+			/* type ID 0 is void, skip it */
+			if (idx == 0)
+				continue;
+
 			err = btf_dump__dump_type(d, idx);
 			if (err)
 				goto done;

tools/testing/selftests/bpf/bpftool_btf_dump_sorted.expected

@@ -21,6 +21,10 @@
 #endif
 #endif
 
+enum {
+	E0 = 1,
+};
+
 struct holey {
 	int c;
 	long: 32;