Set a minimum required version for libaccounts-glib

If it's not found, it will error out with a message that libaccounts- glib 1.21 is required

The reason for this is that KAccounts, the KDE version of accounts-sso implenetation is looking only for libaccounts-qt (and not -glib), so by forcing libaccounts-qt to look for this specific version, there is no need for KAccounts to additionally check for libaccounts-glib version.