gateway-dk-ask: fix xtables extension init — use -D_INIT (uppercase)

The xtables header (xtables.h) has:
  #ifdef _INIT
  #  define _init __attribute__((constructor)) _INIT
  #endif

When -D_INIT=libxt_<name>_init (uppercase) is defined, the header
chains _init → __attribute__((constructor)) _INIT → libxt_<name>_init.
The constructor attribute makes dlopen() automatically call the init
function, which registers the target with xtables_register_target().

Our build was using -D_init (lowercase) which directly renamed the
symbol but bypassed the header's #ifdef _INIT block, so the constructor
attribute was never applied. The extension loaded but _init was never
called, leaving options unregistered.

This matches how iptables itself builds extensions (Makefile.am uses
-D_INIT=lib$*_init).

Author: Tomaz Zaman

extensions/gateway-dk-ask.sh

@@ -283,7 +283,7 @@ function pre_customize_image__001_build_ask_userspace() {
 	chroot_sdcard "cd /tmp/ask-userspace/iptables-extensions && \
 		for name in ${ask_xtables_modules[*]}; do \
 			gcc -shared -fPIC -O2 \
-				-D_init=\${name}_init \
+				-D_INIT=\${name}_init \
 				-I./include \
 				-o \"\${name}.so\" \"\${name}.c\" || exit 1; \
 		done && \