SelectSizeTypeComboBox: Use KiB/MiB/GiB quantifiers for clarity

Summary:
Binary SI units are used in Sieve, see
https://tools.ietf.org/html/rfc5228#section-2.4.1

Test Plan: it compiles

Reviewers: kde-pim, winterz

Reviewed By: winterz

Subscribers: winterz, kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D26025
1 job for master in 10 minutes and 37 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #47577
linux kf5-qt5 qt5.12

00:10:37

56.0%