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>

include/uapi/linux/netfilter/Kbuild

@@ -33,6 +33,7 @@ header-y += xt_NFLOG.h
 header-y += xt_NFQUEUE.h
 header-y += xt_RATEEST.h
 header-y += xt_SECMARK.h
+header-y += xt_SYNPROXY.h
 header-y += xt_TCPMSS.h
 header-y += xt_TCPOPTSTRIP.h
 header-y += xt_TEE.h