grub-install: Fix inverted test for NLS enabled when copying locales
    
    Commit 3d8439da8 (grub-install: Locale depends on nls) attempted to avoid
    copying locale files to the target directory when NLS was disabled.
    However the test is inverted, and it does the opposite.
    
    Signed-off-by: Martin Whitaker <fsf@martin-whitaker.me.uk>
    Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
    Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>