subordinateio: Fix subordinate_parse to have an internal static buffer

subordinateio: Fix subordinate_parse to have an internal static buffer

subordinate_parse is supposed to return a static structure that
represents one line in /etc/subuid or /etc/subgid.  I goofed and
failed to make the variable rangebuf that holds the username of
in the returned structure static.

Add this missing static specification.

Author: <Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>