This is caused by GLib => 2.58 which was uploaded in September 2018 long ago.
Likewise, libsignon-glib 1.12 lags behind upstream substantially. It's 2.1 and the disparity causes the patch to not apply. A cheap workaround might be to add a -Wno-error like is already done for some other deprecated functions. (That has since been fixed upstream proper.) But in version 1.13, the NEWS file says * Build: don't emit a build error on deprecations so perhaps to fix this "cleanly" without such a leap, this avenue may be most suitable (perhaps 1.15).
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