xtables: fix compilation warning

xtables-standalone.c: In function ‘xtables_main’:
xtables-standalone.c:64:2: warning: implicit declaration of function ‘do_commandx’ [-Wimplicit-function-declaration]

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
diff --git a/iptables/nft.h b/iptables/nft.h
index 1bd9ccc..0021668 100644
--- a/iptables/nft.h
+++ b/iptables/nft.h
@@ -62,4 +62,7 @@
  */
 const char *nft_strerror(int err);
 
+/* For xtables.c */
+int do_commandx(struct nft_handle *h, int argc, char *argv[], char **table);
+
 #endif