[BUGFIX] Install Tool: Fix Load Extensions JS error

The checkExtensionsCompatibility() function is not global anymore.

Resolves: #63518
Releases: master
Change-Id: Ie25311b3057c8a7ee988cedce3404dca82af893a
Reviewed-on: http://review.typo3.org/34941
Reviewed-by: Christoph Hofmann <typo3@its-hofmann.de>
Tested-by: Christoph Hofmann <typo3@its-hofmann.de>
Reviewed-by: Jigal van Hemert <jigal.van.hemert@typo3.org>
Tested-by: Jigal van Hemert <jigal.van.hemert@typo3.org>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>