build: fix static linking

build: fix static linking

Gabor Z. Papp noted this link-time error when configuring with
--enable-static:

extensions/libext4.a(initext4.o): In function "init_extensions":
extensions/initext4.c:144: undefined reference to "libxt_IDLETIMER_init"
extensions/initext4.c:145: undefined reference to "libxt_TEE_init"

Indeed, since the two modules did not use our special macro "_init"
(which expands to libxt_foo_init), initext4.c could not find them by
that name. Correct this.

References: http://marc.info/?l=netfilter&m=128085480927924&w=2
Signed-off-by: Jan Engelhardt <jengelh@medozas.de>