libbpf: Fix non-C89 loop variable declaration in gen_loader.c

Fix the `int i` declaration inside the for statement. This is non-C89
compliant. See [0] for user report breaking BCC build.

  [0] libbpf/libbpf#403

Fixes: 18f4fcc ("libbpf: Update gen_loader to emit BTF_KIND_FUNC relocations")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/bpf/20211105191055.3324874-1-andrii@kernel.org

tools/lib/bpf/gen_loader.c

@@ -584,8 +584,9 @@ void bpf_gen__record_extern(struct bpf_gen *gen, const char *name, bool is_weak,
 static struct ksym_desc *get_ksym_desc(struct bpf_gen *gen, struct ksym_relo_desc *relo)
 {
 	struct ksym_desc *kdesc;
+	int i;
 
-	for (int i = 0; i < gen->nr_ksyms; i++) {
+	for (i = 0; i < gen->nr_ksyms; i++) {
 		if (!strcmp(gen->ksyms[i].name, relo->name)) {
 			gen->ksyms[i].ref++;
 			return &gen->ksyms[i];