bpf: Fix build error without CONFIG_NET

If CONFIG_NET is n, building fails:

kernel/trace/bpf_trace.o: In function `raw_tp_prog_func_proto':
bpf_trace.c:(.text+0x1a34): undefined reference to `bpf_skb_output_proto'

Wrap it into a #ifdef to fix this.

Fixes: a7658e1a ("bpf: Check types of arguments passed into helpers")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20191018090344.26936-1-yuehaibing@huawei.com

kernel/trace/bpf_trace.c

@@ -1055,8 +1055,10 @@ raw_tp_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 	switch (func_id) {
 	case BPF_FUNC_perf_event_output:
 		return &bpf_perf_event_output_proto_raw_tp;
+#ifdef CONFIG_NET
 	case BPF_FUNC_skb_output:
 		return &bpf_skb_output_proto;
+#endif
 	case BPF_FUNC_get_stackid:
 		return &bpf_get_stackid_proto_raw_tp;
 	case BPF_FUNC_get_stack: