include: correct re-building conditions around hypercall-defs.h

include: correct re-building conditions around hypercall-defs.h

For a .cmd file to be picked up, the respective target needs to be
listed in $(targets). This wasn't the case for hypercall-defs.i, leading
to permanent re-building even on an entirely unchanged tree (because of
the command apparently having changed).

In exchange the target doesn't need naming in $(clean-files) anymore.

Fixes: eca1f00d0227 ("xen: generate hypercall interface related code")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>