Correct the mirror hash to reflect whats on the download server.
A locally produced libubox SCM tarball was also verified to yield an identical
checksum compared to the one currently on the download server.
Fixes FS#1707.
Fixes 5dc32620c4 ("libubox: update to latest git HEAD")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>