snowball-init: add .hash file
Fixes:
http://autobuild.buildroot.org/results/fba/fba69e25de9463282094bcd9b18b63ba6de0a892/
Similar to jquery-keyboard, it seems the snowball-init tarball on the
autobuilder is corrupt. Fix it by adding a .hash file so it falls back to
our s.b.o mirror.
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>