A typo was meaning that the mirror creation method wasn't being called
when it should have been. Fix the type to fix mirror tarball creation.
[YOCTO #5284]
(Bitbake rev: 348971d410bfd5d8b1757468d73e1d24ae78a594)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>