xtables: fix compilation due to missing autogenerated header

Fix compilation:

nft.c:51:35: fatal error: xtables-config-parser.h: File or directory doesn't exist

xtables-config-parser.h was generated after compiling nft.c.

Reported-by: Giuseppe Longo <giuseppelng@gmail.com>
Tested-by: Giuseppe Longo <giuseppelng@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
diff --git a/iptables/Makefile.am b/iptables/Makefile.am
index 5d65354..5cbd8ff 100644
--- a/iptables/Makefile.am
+++ b/iptables/Makefile.am
@@ -28,10 +28,10 @@
 if ENABLE_NFTABLES
 if HAVE_LIBMNL
 if HAVE_LIBNFTABLES
+xtables_multi_SOURCES += xtables-config-parser.y xtables-config-syntax.l
 xtables_multi_SOURCES += xtables-save.c xtables-restore.c \
 			 xtables-standalone.c xtables.c nft.c \
 			 nft-shared.c nft-ipv4.c nft-ipv6.c \
-			 xtables-config-parser.y xtables-config-syntax.l \
 			 xtables-config.c xtables-events.c
 xtables_multi_LDADD   += -lmnl -lnftables ${libmnl_LIBS} ${libnftables_LIBS}
 xtables_multi_CFLAGS  += -DENABLE_NFTABLES