Revert "staging: wfx: remove unused included header files"

This reverts commit 314fd52f.

It turns .h files into non-stand-alone when building, which might cause
problems in the long-run.

Reported-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Cc: Muhammad Usama Anjum <musamaanjum@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>