package/mtd-utils: fix git build after gitweb update

Reported by Daniel Mack <daniel@caiaq.de>

The gitweb installation of infradead.org has been updated, and gitweb >=
1.6.6-rc1 adds the first 7 characters of the git SHA1 to the snapshot
tarball directory name.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>