scripts/headers_install.sh: fix error handling

- headers_install requires at least two arguments

- missed closing quote

Signed-off-by: Javier Barrio <javier.barrio.mart@gmail.com>
Cc: Michal Marek <mmarek@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>