bpf: Fix "unresolved symbol" build error with resolve_btfids

Michal reported a build failure likes below:

   BTFIDS  vmlinux
   FAILED unresolved symbol tcp_timewait_sock
   make[1]: *** [/.../linux-5.9-rc7/Makefile:1176: vmlinux] Error 255

This error can be triggered when config has CONFIG_NET enabled
but CONFIG_INET disabled. In this case, there is no user of
istructs inet_timewait_sock and tcp_timewait_sock and hence
vmlinux BTF types are not generated for these two structures.

To fix the problem, let us force BTF generation for these two
structures with BTF_TYPE_EMIT.

Fixes: fce557bc ("bpf: Make btf_sock_ids global")
Reported-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20201001051339.2549085-1-yhs@fb.com