package/opencv3: add .hash file

Similar to jquery-keyboard, it seems the opencv3 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: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/opencv3/opencv3.hash

@@ -0,0 +1,2 @@
+# Locally calculated
+sha256 f00b3c4f42acda07d89031a2ebb5ebe390764a133502c03a511f67b78bbd4fbf  opencv3-3.1.0.tar.gz