Fix installation of icons

Summary:
Without this patch, only svg icons would end up being installed as
the file(GLOB ...) line overwrote the previous content of ${icons}.

Test Plan: Some debug output with message(), did not do a full build.

Reviewers: mlaurent, dvratil

Reviewed By: dvratil

Subscribers: #kde_pim_kmail, #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D5055