Fix missing global variable declaration

The wgCentralAuthCheckSULMigration variable is used in
CentralAuthHooks::onRunExtensionFunctions without a corresponding
global declaration, which breaks, for example, QUnit tests for
extensions that depend on CentralAuth [0].

[0] https://integration.wikimedia.org/ci/job/mwext-GettingStarted-qunit/413/console

Change-Id: I425fe8d9d5a30ef0eb500d8d812d7e252f3a81ad