iptables: fix nftables compile issue (FS#711)

iptables: fix nftables compile issue (FS#711)

Enabling IPTABLES_NFTABLES resulted in an error during build:#
*** No rule to make target '../extensions/libext.a',
needed by 'xtables-compat-multi'."

Comments from Alexander Lochmann and Fedor Konstantinov in FS#711
provided fixes for this build error, allowing iptables to compile.
https://bugs.lede-project.org/index.php?do=details&task_id=711.

This commit updates the Makefile.am xtables_compat_multi_LDFLAGS
and _LDADD, moving linking of extensions to LDFLAGS.

Signed-off-by: rektide de la faye <rektide@voodoowarez.com>
Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>