level: Fix the spectrum example for 1.0

level: Fix the spectrum example for 1.0

The "message" property has been replaced by "post-messages".

Pre-patch output:

(test_spectrum:23101): GLib-GObject-WARNING **: g_object_set_valist:
object class `GstSpectrum' has no property named `message'
New spectrum message, endtime 0:00:00.100000000

(test_spectrum:23101): GStreamer-CRITICAL **:
gst_value_list_get_value: assertion `GST_VALUE_HOLDS_LIST (value)' failed
[...]

Post-patch:

New spectrum message, endtime 0:00:00.100000000
band 0 (freq 400): magnitude -65.988777 dB phase 1.533397
band 1 (freq 1200): magnitude -65.545563 dB phase -0.780900
band 2 (freq 2000): magnitude -64.791946 dB phase -0.799611
band 3 (freq 2800): magnitude -64.556175 dB phase -0.063615
[...]

https://bugzilla.gnome.org/show_bug.cgi?id=704179