Add missing KF5WindowSystem dependency

Added in commit a45bfe36 w/o searching for it.

Signed-off-by: Andreas Sturmlechner <asturm@gentoo.org>
1 job for !413 with work/missing-dep in 4 minutes and 35 seconds (queued for 35 minutes and 50 seconds)
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passed linux kf5-qt5 qt5.15 #118445

00:04:35

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