kbx: Fix for building without keyboxd.

* kbx/keybox-fwddecl.h: New.
* kbx/keybox.h: Replace typedef for KEYBOX_HANDLE by including the new
file.
* g10/keydb-private.h: Ditto.

The duplicated typedef was a bit ugly and will fail, depending on
compiler, iof for example building without keyboxd. Fix only tested
in the standard case but the fix is obvious.