smb client: Add generated file to gitignore file

The smb client code recently started generating the error mapping table
from a common header, but didn't tell git about it, so then git ends up
thinking maybe it should be committed.

Let's fix that.

Fixes: c527e13a7a66 ("cifs: Autogenerate SMB2 error mapping table")
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>