net: wifi: Add iterable_sections header file

This is related to change in commit dacb3dbf
("iterable_sections: move to specific header")

Until now iterable sections APIs have been part of the toolchain
(common) headers. They are not strictly related to a toolchain, they
just rely on linker providing support for sections. Most files relied on
indirect includes to access the API, now, it is included as needed.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>