cargo: add version number to jj-lib dependency

This is required for `cargo publish`. It seems better to have it
tracked than to do add it whenever we publish.

Cargo.toml

@@ -94,7 +94,7 @@ zstd = "0.12.4"
 # put all inter-workspace libraries, i.e. those that use 'path = ...' here in
 # their own (alphabetically sorted) block
 
-jj-lib = { path = "lib" }
+jj-lib = { path = "lib", version = "0.9.0" }
 testutils = { path = "lib/testutils" }
 
 [profile.release]