rustc: fix check-package warnings

utils/check-package complains as follows:

  package/rustc/Config.in.host:6: attributes order: type, default, depends on, select, help (http://nightly.buildroot.org/#_config_files

)
  package/rustc/Config.in.host:79: empty line at end of file

This patch fixes these warnings.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>