Makefile: fix static compilation iptables-compat without shared libraries

Makefile: fix static compilation iptables-compat without shared libraries

Fix linking problems when this configuration is used:

 $ ./configure --prefix=/usr --enable-shared=no --enable-static=yes
 $ make
 ...
xtables_compat_multi-xtables-save.o: In function `xtables_save_main':
/home/pablo/devel/iptables/iptables/xtables-save.c:98: undefined reference to `init_extensions4'
xtables_compat_multi-xtables-restore.o: In function `xtables_restore_main':
/home/pablo/devel/iptables/iptables/xtables-restore.c:195: undefined reference to `init_extensions4'
xtables_compat_multi-xtables-standalone.o: In function `xtables_main':
/home/pablo/devel/iptables/iptables/xtables-standalone.c:61: undefined reference to `init_extensions4'
xtables_compat_multi-xtables-events.o: In function `xtables_events_main':
/home/pablo/devel/iptables/iptables/xtables-events.c:184: undefined reference to `init_extensions4'

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>