bpf_helpers_doc.py: Fix warning when compiling bpftool

When compiling bpftool the following warning is found: "declaration of
'struct bpf_pidns_info' will not be visible outside of this function."
This patch adds struct bpf_pidns_info to type_fwds array to fix this.

Fixes: b4490c5c

 ("bpf: Added new helper bpf_get_ns_current_pid_tgid")
Signed-off-by: Carlos Neira <cneirabustos@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20200313154650.13366-1-cneirabustos@gmail.com