netfilter: xtables: remove extraneous header that slipped in

Commit 0b8ad876 (netfilter: xtables: add missing header files to export
list) erroneously added this.
Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
Signed-off-by: Patrick McHardy <kaber@trash.net>

include/linux/netfilter/Kbuild

@@ -56,7 +56,6 @@ header-y += xt_rateest.h
 header-y += xt_realm.h
 header-y += xt_recent.h
 header-y += xt_sctp.h
-header-y += xt_secmark.h
 header-y += xt_socket.h
 header-y += xt_state.h
 header-y += xt_statistic.h