build: include <net/ip6_checksum.h> for csum_ipv6_magic

xt_ECHO fails to build on PPC because csum_ipv6_magic is declared in
<net/ip6_checksum.h>, which is not implicitly included from other
headers on PPC causing build failures due to this function being
undefined. So, include this header explicitly.

Note:  Same cause as <http://bugzilla.netfilter.org/show_bug.cgi?id=307>.

extensions/xt_ECHO.c

@@ -18,6 +18,7 @@
 #	include <linux/netfilter_bridge.h>
 #endif
 #include <net/ip.h>
+#include <net/ip6_checksum.h>
 #include <net/ip6_route.h>
 #include <net/route.h>
 #include "compat_xtables.h"