meson.build: Fix glib -Wno-unused-function workaround

We want to only enable '-Wno-unused-function' if glib's version is
smaller than '2.57.2' and has a G_DEFINE_AUTOPTR_CLEANUP_FUNC()
implementation that doesn't take into account unused functions. But the
compilation test isn't working as intended as '-Wunused-function' isn't
enabled while running it.

Let's enable it.

Fixes: fc9a809e ("build: move glib detection and workarounds to meson")
Signed-off-by: Nicolas Saenz Julienne <nsaenz@amazon.com>

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230524173123.66483-1-nsaenz@amazon.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>