Mark the dixChangeWindowProperty() value argument as const

It is copied using memcpy() and not modified so we can add const. This
fixes a -Wincompatible-pointer-types-discards-qualifiers compiler warning
that was failing a -Werror XVnc build for me.

Signed-off-by: Alex Richardson <Alexander.Richardson@cl.cam.ac.uk>
8 jobs for add-const-to-function-signature in 6 minutes and 33 seconds (queued for 8 seconds)
Status Name Job ID Coverage
  Docker Image
passed debian-buster #12341167

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  Build And Test
passed autotools #12341168

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passed meson #12341169

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passed meson-noglamor #12343573

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passed mingw-cross-build #12341171

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failed meson-noglamor #12341173

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failed meson-noglamor #12341170

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failed meson-noglamor #12341172

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