GtkBuildable: Fix the type of the user_data in GtkBuildable.custom_tag_end

The previous type was a pointer to a pointer, which seems to be a copy-paste
error from GtkBuildable.custom_tag_start which is an out parameter. It was
always cast in use so this is an API break, but not an ABI one.
2 jobs for gtkbuilder-tag-end-wrong-type-3-24 in 7 minutes and 53 seconds (queued for 27 minutes and 19 seconds)
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Status Job ID Name Coverage
  Build
passed #118891
build

00:07:53

manual #118892
win32 allowed to fail manual
msys2-mingw32