selftests/bpf: Fix compilation errors

selftests/bpf: Fix compilation errors

If the CONFIG_NET_SCH_BPF configuration is not enabled,
the BPF test compilation will report the following error:
In file included from progs/bpf_qdisc_fq.c:39:
progs/bpf_qdisc_common.h:17:51: error: declaration of 'struct bpf_sk_buff_ptr' will not be visible outside of this function [-Werror,-Wvisibility]
   17 | void bpf_qdisc_skb_drop(struct sk_buff *p, struct bpf_sk_buff_ptr *to_free) __ksym;
      |                                                   ^
progs/bpf_qdisc_fq.c:309:14: error: declaration of 'struct bpf_sk_buff_ptr' will not be visible outside of this function [-Werror,-Wvisibility]
  309 |              struct bpf_sk_buff_ptr *to_free)
      |                     ^
progs/bpf_qdisc_fq.c:309:14: error: declaration of 'struct bpf_sk_buff_ptr' will not be visible outside of this function [-Werror,-Wvisibility]
progs/bpf_qdisc_fq.c:308:5: error: conflicting types for '____bpf_fq_enqueue'

Fixes: 11c701639ba9 ("selftests/bpf: Add a basic fifo qdisc test")
Signed-off-by: Feng Yang <yangfeng@kylinos.cn>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20250428033445.58113-1-yangfeng59949@163.com