cifs: In Kconfig CONFIG_CIFS_POSIX needs depends on legacy (insecure cifs)

Missing a dependency.  Shouldn't show cifs posix extensions
in Kconfig if CONFIG_CIFS_ALLOW_INSECURE_DIALECTS (ie SMB1
protocol) is disabled.

Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>