Fix SqueezerBin::size_allocate() in example

[mtilda]

The example squeezer_bin is not working on my machine, as described in #1757.

While investigating that bug, I noticed a mistake in SqueezerBin::size_allocate(). I noticed the function child_size() is returning all zeros, because it is being passed the wrong argument. This PR fixes that mistake.

There is still a bug in the code that causes this example to panic. To avoid panic and to test this change, comment out these lines:

gtk4-rs/examples/squeezer_bin/main.rs

Lines 22 to 24 in 3826b9a

	mode_switch
	.bind_property("state", &squeezer, "keep-aspect-ratio")
	.build();

cd into the directory examples and run cargo run --bin squeezer_bin.

After this change, the text "Hello World!" should be visible in the window.