bpf: fix script for generating man page on BPF helpers

The script broke on parsing function prototype for bpf_strtoul(). This
is because the last argument for the function is a pointer to an
"unsigned long". The current version of the script only accepts "const"
and "struct", but not "unsigned", at the beginning of argument types
made of several words.

One solution could be to add "unsigned" to the list, but the issue could
come up again in the future (what about "long int"?). It turns out we do
not need to have such restrictions on the words: so let's simply accept
any series of words instead.

Reported-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>