Correct installation of start_kdeinit when DESTDIR and libcap are used together

Summary:
While packaging kinit 5.38.0 for Adélie, I ran in to the following error:

-- Installing: /usr/src/packages/user/kinit/pkg/kinit/usr/lib/libexec/kf5/start_kdeinit
Failed to set capabilities on file `/usr/lib/libexec/kf5/start_kdeinit' (No such file or directory)
usage: setcap [-q] [-v] (-r|-|<caps>) <filename> [ ... (-r|-|<capsN>) <filenameN> ]

 Note <filename> must be a regular (non-symlink) file.

This was caused by a missing escape in the CMake instruction to run setcap. I have verified that this patch also applies cleanly to master.

Test Plan: * Build with and without libcap, with and without DESTDIR set.

Reviewers: dfaure

Reviewed By: dfaure

Subscribers: graesslin, Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D8064