netfilter: xtables: add missing header files to export list

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>

include/linux/netfilter/Kbuild

@@ -56,6 +56,8 @@ 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
 header-y += xt_string.h