headers_install: check exit status of unifdef

If unifdef fails for any reason (like segfaulting), we should be aborting
the install steps.  So check its exit status in this unlikely scenario.

Reported-by: Diego Elio Pettenò <flameeyes@gentoo.org>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>