Make Documenation depend on headers_install

Cc: rdunlap@infradead.org
Cc: linux-doc@vger.kernel.org
Cc: sudeep.dutt@intel.com
Cc: nikhil.rao@intel.com
Cc: ashutosh.dixit@intel.com
Cc: akpm@linux-foundation.org
Cc: gregkh@linuxfoundation.org
Signed-off-by: Peter Foley <pefoley2@pefoley.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>