Fix -Wunused-variable

Summary:
Compiler warning message:
    [524/1591] Building CXX object CMakeFiles/kwin.dir/workspace.cpp.o
    /home/vlad/Workspace/KDE/src/kde/workspace/kwin/workspace.cpp: In lambda function:
    /home/vlad/Workspace/KDE/src/kde/workspace/kwin/workspace.cpp:233:47: warning: unused variable ‘otherDesktop’ [-Wunused-variable]
                             const VirtualDesktop *otherDesktop = VirtualDesktopManager::self()->desktops().first();
                                                   ^~~~~~~~~~~~

Test Plan: Compiles.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D17060