tests: http_server: Add missing zephyr_iterable_section

As the test uses iterable sections, we need to utilize
zephyr_iterable_section for targets that need linker script
generation like arm-clang.

So add zephyr_iterable_section() for the ROM sections that the
testcases utilizes.

Signed-off-by: Kumar Gala <kumar.gala@intel.com>