iwlwifi: add required include to iwl-fh.h

Some variables are used in iwl-fh.h, but their type's definition
isn't included. This may break compilation when using the file
without including the necessary dependencies first.

Add the required include to iwl-fh.h.

Signed-off-by: Golan Ben Ami <golan.ben.ami@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>