mptcp, btf: Add struct mptcp_sock definition when CONFIG_MPTCP is disabled

The btf_sock_ids array needs struct mptcp_sock BTF ID for the
bpf_skc_to_mptcp_sock helper.

When CONFIG_MPTCP is disabled, the 'struct mptcp_sock' is not
defined and resolve_btfids will complain with:

  [...]
  BTFIDS  vmlinux
  WARN: resolve_btfids: unresolved symbol mptcp_sock
  [...]

Add an empty definition for struct mptcp_sock when CONFIG_MPTCP
is disabled.

Fixes: 3bc253c2 ("bpf: Add bpf_skc_to_mptcp_sock_proto")
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20220802163324.1873044-1-jolsa@kernel.org

include/net/mptcp.h

@@ -291,4 +291,8 @@ struct mptcp_sock *bpf_mptcp_sock_from_subflow(struct sock *sk);
 static inline struct mptcp_sock *bpf_mptcp_sock_from_subflow(struct sock *sk) { return NULL; }
 #endif
 
+#if !IS_ENABLED(CONFIG_MPTCP)
+struct mptcp_sock { };
+#endif
+
 #endif /* __NET_MPTCP_H */