netfilter: xt_SYNPROXY: add missing header to Kbuild

Matt Whitlock says:

 Without this line, the file xt_SYNPROXY.h does not get installed in
 /usr/include/linux/netfilter/, and thus user-space programs cannot make
 use of it.

Reported-by: Matt Whitlock <kernel@mattwhitlock.name>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>