Add missing Qt5Concurrent to target_link_libraries

This was working as long as it was implicitly provided by KIO.

See also: frameworks/kio!426

Signed-off-by: Andreas Sturmlechner <asturm@gentoo.org>
(cherry picked from commit 3f25d720)
2 jobs for master in 8 minutes and 50 seconds (queued for 4 minutes and 13 seconds)
Status Name Job ID Coverage
  Build
passed flatpak #116994
flatpak

00:06:37

passed linux kf5-qt5 qt5.15 #116993

00:02:13

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