[BUGFIX] Explicit require dbal autoloader file

The dbal autoload file in Installer.php may not be found if
ext:dbal is not loaded. This is solved with an explicit require.

Change-Id: I9991d579ec56c85dd117bc7938d129844b2d3150
Resolves: #47847
Related: #47836
Releases: 6.2
Reviewed-on: https://review.typo3.org/20440
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn