[BUGFIX] Deactivating "install" extension leads to exceptions

Uninstalling the "install" extension leads to exceptions in
required system extensions like extensionmanager or lang.

The solution is to make "install" a required extension too.

Change-Id: Id5a0ebd9400648cd7c453d98b58e8cad1d155bd8
Fixes: #43361
Releases: 6.1, 6.0
Reviewed-on: https://review.typo3.org/18802
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn