(urgent) .gitmodules: fix xcb-util-m4 url and CI failure

URGENT: CI isn't capable of building debian builder images anymore, since cloning this repo fails.

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anongit.freedesktop.org doesn't seem to work anymore - using the canonical gitlab https URL instad, so making the CIs work again.

Signed-off-by: Enrico Weigelt, metux IT consult info@metux.net