[BUGFIX] Fix position of search field clearer in extension manager.

The jquery plugin is not a real external contribution, but was written
specifically for TYPO3, so modifying it in the contrib-folder should be
ok.

Change-Id: I263d827b8e08a83c345b3525c53124483eba5620
Resolves: #57051
Releases: 6.2
Reviewed-on: https://review.typo3.org/28659
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn