libbpf: Workaround (another) -Wmaybe-uninitialized false positive

libbpf: Workaround (another) -Wmaybe-uninitialized false positive

We get this with GCC 15 -O3 (at least):
```
libbpf.c: In function ‘bpf_map__init_kern_struct_ops’:
libbpf.c:1109:18: error: ‘mod_btf’ may be used uninitialized [-Werror=maybe-uninitialized]
 1109 |         kern_btf = mod_btf ? mod_btf->btf : obj->btf_vmlinux;
      |         ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
libbpf.c:1094:28: note: ‘mod_btf’ was declared here
 1094 |         struct module_btf *mod_btf;
      |                            ^~~~~~~
In function ‘find_struct_ops_kern_types’,
    inlined from ‘bpf_map__init_kern_struct_ops’ at libbpf.c:1102:8:
libbpf.c:982:21: error: ‘btf’ may be used uninitialized [-Werror=maybe-uninitialized]
  982 |         kern_type = btf__type_by_id(btf, kern_type_id);
      |                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
libbpf.c: In function ‘bpf_map__init_kern_struct_ops’:
libbpf.c:967:21: note: ‘btf’ was declared here
  967 |         struct btf *btf;
      |                     ^~~
```

This is similar to the other libbpf fix from a few weeks ago for
the same modelling-errno issue (fab45b962749184e1a1a57c7c583782b78fad539).

Signed-off-by: Sam James <sam@gentoo.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://bugs.gentoo.org/939106
Link: https://lore.kernel.org/bpf/f6962729197ae7cdf4f6d1512625bd92f2322d31.1725630494.git.sam@gentoo.org