Constrain libsmbclient bug workaround to affected versions only

Summary:
A bug where libsmbclient returned nonsensical error code which prevented kio_smb plugin to query user for login credentials appears to have been fixed in Samba 4.7.7 and Samba 4.8.0. Constrain the workaround for the problem only to affected version of libsmbclient.

Samba 4.7.7 release notes: https://www.samba.org/samba/history/samba-4.7.7.html
Relevant bug report: https://bugzilla.samba.org/show_bug.cgi?id=13050

CCBUG: 390551

Reviewers: elvisangelaccio, ngraham

Reviewed By: elvisangelaccio, ngraham

Differential Revision: https://phabricator.kde.org/D12798