The default values for construct properties always have to be set, even
if those properties are deprecated. The code to do that is in GLib, and
not under the control of the user (unless they completely override the
`constructor` vfunc, which is not recommended). So don’t emit a warning
for that if `G_ENABLE_DIAGNOSTICS` is enabled.

In particular, this fixes deprecation warnings being emitted for
properties of a parent class when chaining up with a custom constructor,
even when none of the child class code mentions the deprecated property.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Fixes: #3254