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] https://github.com/libbpf/libbpf/issues/403

Fixes: 18f4fccb

 ("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