jquery.accessKeyLabel: Enable for target=mobile

Follows-up 6a81b0e, which split this module out of mediawiki.util,
but didn't give it the same target. So the mediawiki.util module
was now broken on module due to a dependency on an unknown module
named "jquery.accessKeyLabel".

Bug: 64512
Change-Id: I324501411829a71e30d424a8c5304748575fcba3