cjdns: pass for some build warnings (#738)

cjdns: pass for some build warnings (#738)

Compile time changes reviewed by cjd.
Due to stricter compiler(s) we do not
want to error for these warnings.

Caused by: ...reading "past the end"
of a struct, because the struct is
actually a header and the body is of
unknown size. Compilers got stricter
and this became a warning.

Signed-off-by: William Fleurant <meshnet@protonmail.com>
(cherry picked from commit 71c11ab1b7d7f6ca3219c1359e20b168ec1979c5)
[Fix compiling]
Signed-off-by: Nick Hainke <vincent@systemli.org>