Use non-const iterators

Summary:
on some GCC (here 7.5.0) it complains when one tries to reassign an
iterator that was obtained via const_find, using find() using a
mutable iterator makes it build fine

Test Plan: Builds, behavior not changed

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D28497