opencv: add .hash file
Fixes:
http://autobuild.buildroot.org/results/e06/e06f0e6edd5d8013ff5e12e2b7253e99a1d1fa2e/
Similar to jquery-keyboard, it seems the opencv 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>