base.bbclass: pull in file-native for src.rpm

Unpacking an src.rpm uses rpm2cpio.sh, which requires 'file'.

Without this, builds of rpm on a host without 'file' installed will fail with
very strange messages.

Signed-off-by: Christopher Larson <kergoth@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>