Add .cargo to gitignore (#541)

To use a specific version of MLIR, it is useful to set the corresponding
environment variable in `.cargo/config.toml`. In order to use this while
working in melior, I think it makes sense to gitignore the `.cargo`
directory. This can be reverted if the project ever needs to set it to
something specific.

Co-authored-by: Yota Toyama <[email protected]>

.gitignore

@@ -1 +1,2 @@
 target
+.cargo