Fix #64382: Add pointer cursor to search clear button

[manhphuc]

This PR addresses ticket #64382 in WordPress Trac.

The issue:
When using the search input in the Posts list screen (/wp-admin/edit.php),
the search clear (×) button does not display a cursor: pointer on hover,
making it appear non-interactive even though it is clickable.

The fix:
Adds cursor: pointer to the WebKit search input clear button pseudo-element.

Testing:

• Verified on Chrome, Safari, and Firefox.
• Hover state now correctly shows a pointer cursor.
• No regressions observed.

Trac ticket: https://core.trac.wordpress.org/ticket/64382

Note:
Per WordPress Core guidelines, this PR is provided for code review only.
All discussion and final commit should happen in the Trac ticket.