[BUGFIX] Allow proper method override in tx_coreupdates_installsysexts

Method tx_coreupdates_installsysexts::performUpdate() cannot be properly overriden
because its signature is incomplete.

This leads to PHP notices.

Change-Id: Iaf13c166c07d04ee194bec1543ac5ae450f1e38b
Fixes: #30053
Releases: 4.6
Reviewed-on: http://review.typo3.org/5078
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers