net: libwx: Replace zero-length array with flexible-array member

Zero-length arrays as fake flexible arrays are deprecated, and we are
moving towards adopting C99 flexible-array members instead.

Transform zero-length array into flexible-array member in struct
wx_q_vector.

Link: https://github.com/KSPP/linux/issues/21
Link: https://github.com/KSPP/linux/issues/286
Link: https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/ZGKGwtsobVZecWa4@work
Signed-off-by: Jakub Kicinski <kuba@kernel.org>