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 (fab45b96).
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

tools/lib/bpf/libbpf.c

@@ -986,7 +986,7 @@ find_struct_ops_kern_types(struct bpf_object *obj, const char *tname_raw,
 {
 	const struct btf_type *kern_type, *kern_vtype;
 	const struct btf_member *kern_data_member;
-	struct btf *btf;
+	struct btf *btf = NULL;
 	__s32 kern_vtype_id, kern_type_id;
 	char tname[256];
 	__u32 i;
@@ -1116,7 +1116,7 @@ static int bpf_map__init_kern_struct_ops(struct bpf_map *map)
 	const struct btf *btf = obj->btf;
 	struct bpf_struct_ops *st_ops;
 	const struct btf *kern_btf;
-	struct module_btf *mod_btf;
+	struct module_btf *mod_btf = NULL;
 	void *data, *kern_data;
 	const char *tname;
 	int err;